A Multi-Stimuli-Responsive Oxazine Molecular Switch: A Strategy for the Design of Electrochromic Materials

Article abstract of DOI:10.1002/asia.201800282

A multi-state and multi-stimuli-responsive oxazine molecular switch that combines an electro-base property and sensitive base/acid-responsive properties was designed and synthesized. The multi-state structures of the molecular switch, with different colors, were predicted by comparing the optical properties with reference molecules and confirmed by using NMR spectroscopy. The color-switching mechanism under stimulation with acids and bases was investigated by using DFT calculations. Three single states can be obtained and the switching is unidirectional under acid and base stimulation. The electrochromic phenomenon of the molecular switch, which combines its electro-base and base-sensitive properties, was demonstrated. An electrochromic device that exhibited good electrochromic properties with excellent reversibility (2000 cycles) and high coloration efficiency (804 cm² C^?1) was successfully constructed.

Full text of DOI:10.1002/asia.201800282

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: A multi-stimuli-responsive oxazine molecular switch: A new
strategy for the design of electrochromic materials
Authors: Xiaojun Wang, Chang Gu, Hongzhi Zheng, Yu-Mo Zhang, and
Sean Xiao-An Zhang
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To be cited as: Chem. Asian J. 10.1002/asia.201800282
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A multi-stimuli-responsive oxazine molecular switch: A new
strategy for the design of electrochromic materials
Xiaojun Wang, Chang Gu, Hongzhi Zheng, Yu-Mo Zhang* and Sean Xiao-An Zhang*
Abstract:
A
multi-state and multi-stimuli-responsive oxazine
molecular switches exhibits wide variety, high purity, and ideal
molar absorption coefficient. However, the application of pH-
sensitive molecular switches in ECM is hindered for the lack of
ability of the electrical response.[42-44] The ‘‘electro-acid/base”
was utilized by our group to extend the application of pH-
sensitive molecular switches into ECM.[45-47] Electro-acid/base is
molecular switch combining electro-base property and sensitively
base/acid responsive properties was designed and synthesized. The
multi-state structures of the molecular switch with different colors
were speculated by comparison the optical properties with reference
molecules and confirmed by NMR. The color switching mechanism
under the stimulation of acid and base was investigated by DFT
calculation. Three single states can be obtained and the switching is
unidirectional under the stimulation of acid and base. The
electrochromic phenomenon of the molecular switch was
demonstrated combining its electro-base property and base-
sensitive property. An electrochromic device was successfully
constructed which exhibits good electrochromic properties with
excellent reversibility (2000 cycles) and high coloration efficiency
(804 cm2/C).
a
type of molecule (such as p-phenylenediamine and
benzoquinone, etc.) which oxidative or reductive state exhibits
capable of capturing/losing a proton. Compared with chemical
acid/base, the acidity/alkalinity of electro-acid/base could be
controlled by an external electric field in a closed system.
Through utilizing these electro-acid/base molecules, various pH-
sensitive molecular switches have the opportunity to application
in ECM.
In this paper, a oxazine derivate (M−c, Figure 1a) was designed
and synthesized. First, its pH-sensitive properties were studied
systematically with a dual response to acid and base, and the
structures of different states were speculated by comparing the
optical properties with reference molecules and confirmed by
NMR. The switching mechanism of M−c under the stimulation of
acid/base was analyzed by DFT calculation. Then, the electro-
base mechanism of M−c was demonstrated, and an
electrochromic device was fabricated with good electrochromic
properties.
Introduction
Electrochromic materials (ECM), characterized by the tunable
color being switched by electrical field,[1,2] have been attracting
much attention in the past few decades due to their wide
application potential,[3] such as smart windows,[4-7] auto rearview
mirrors, reflect displays,[8-10] etc.[1,11] At present, metal oxides
(WO3),[5,11] conducted polymers[12-14] and viologen derivatives[15]
are the three most intensively researched kinds of ECM. Even
though all these materials exhibit unique advantages and
attractive application potential in electrochromic devices, for
example good photostability of WO3, high color efficiency of
conducted polymer and high reversibility of viologen.[13,16,17]
There are still many problems such as slow response speed and
difficult manufacturing processes for WO3, few color choices for
viologen, and poor color purity for conducted polymer.[16,18-20]
Many efforts have been made to solve these problems.[18,21-25]
And one promising way to solve these problems is exploiting
new electrochromic materials.[26-33]
Results and Discussion
The acid/base-responsive measurement in solution. The
molecule M−c was designed by combining an acid responsive
heterocyclic ring and a base responsive hydroxyl. And two
reference molecules, M−r−1 and M−r−2, were also synthesized
with similar structure with M-c as shown in Figure 1a. To
investigate if the molecule M−c has the acid and base
responsive properties respectively, the UV−vis absorption
spectra of M−c with the addition of acid and base were
measured. As shown in Figure 1b, with increasing concentration
of the trifluoroacetic acid (TFA), the original absorption peak at
267 nm decreases, and a new absorption band at 436 nm
emerges and increases. Only 0.5 equiv of the TFA was needed
to stimulate its structural alteration. And it can be transformed
into acid-form (M−a−o) completely under 5.0 equiv of TFA.
Which demonstrate the M−c has good acid sensitivity. The M−c
molecule still possessed good acid responsive reversibility. The
absorption peak at 436 nm of M−a−o disappears immediately by
adding the same amount of potassium tert-butoxide (t-BuOK) to
neutralize the acid.
Molecular switches represent
a kind of molecule whose
properties such as optical properties, polarity and solubility can
be changed when the structure of the molecule undergoes
transformation under external stimuli such as light, heat, pH,
force and electric field etc.[34-38] Among them, the pH-sensitive
molecular switches are most abundant and have wide
application in pH indicator, biosensing,[39] information
communication etc.[40,41] More importantly, color of pH-sensitive
X. Wang, C. Gu, H. Zheng, Dr. Y. M. Zhang and Prof. S. X. A. Zhang
State Key Lab of Supramolecular Structure and Materials, College of
Chemistry
Jilin University
2699 Qianjin Street, Changchun 130012, P. R. China.
E-mail: zhangyumo@jlu.edu.cn and seanzhang@jlu.edu.cn
Supporting information for this article is given via a link at the end of the
document.
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electron dipole on the p-nitrophenol as a non-conjugated
substituent.
To understand the structural change of M−c in base-treating
process, base responsive properties of M−r−1, M−r−2 and p-
nitrophenol were also investigated. For the M−r−1, there is no
change of the UV−vis spectrum after adding 5 equiv t-BuOK as
shown in Figure S4. It declares that the hydroxyl group is the
important functional group for M−c molecule. The base
responsive properties of M−r−2 are studied as shown in Figure
S2. With the concentration of t-BuOK increasing, the absorption
peak at 424 nm decreases to zero, and the absorption peaks at
538 nm and 507 nm increase. Those peaks are similar with the
absorption peaks at 545 nm and 513 nm for the M−c molecule,
and only
7 nm and 8 nm differences are exhibited. It
demonstrates that the base form of M−c has similar structure
with the base form of M−r−2. Hereinto, the hydroxyl of M−r−2 is
changed to oxygen anion after t-BuOK being added, which can
be proposed readily (Scheme S1a). Only 3 nm difference was
exhibited between the absorption peaks for 432 nm of base-form
M−c and 429 nm of base-form p-nitrophenol (Figure S3).
Therefore, after stimulated by t-BuOK, the M−c molecule should
undergo a proton loss, and then the ring-opening process of
heterocyclic ring to generate a large conjugated part as shown
as M−b−o in Figure 1e, similar to base form of M−r−2. A 109 nm
red shift of the absorption peak position from 436 nm (M−a−o) to
545 nm (M−b−o) is produced by the property of the oxygen
anion as stronger electron donor compared with hydroxyl. The
absorption peaks of 545 nm and 513 nm are supposed to come
from trans-cis isomerism of the double bond on the conjugated
part. Furthermore, molar absorption coefficients of M−b−o (545
nm, ε=9.677×104 L·mol−1·cm−1) is also obviously higher than
M−a−o (ε=4.207×104 L·mol−1·cm−1) because of the stronger D−A
effect (Figure S5).
Figure 1. (a) The chemical structures of M−c molecule and the reference
molecules, M−r−1 and M−r−2. (b) UV−vis absorption spectra of M−c (1.0×
10−5 M) in acetonitrile, adding different equiv TFA, then neutralizing with t-
BuOK. (c) UV−vis absorption spectra of M−c (1.0×10−5 M) in acetonitrile,
adding different equiv t-BuOK, then neutralizing with TFA. (d) The color
changing pictures for M−c solution in acetonitrile stimulated reversibly by the
acid and base. (e) The structure speculation for the acid-form and base-form
of M−c.
The base measurement is shown in Figure 1c. With increasing
base concentration, the absorption peak at 267 nm decreases,
and the absorption peaks at 432 nm, 513 nm and 545 nm
increase. Just 3.0 equiv t-BuOK was needed to achieve the
maximum absorbance peak at 545 nm, along with transforming
M−c to base form (M−b−o) completely. Which indicates the M−c
molecule with good base sensitivity. And then adding 3.0 equiv
TFA to neutralize the t-BuOK, the absorption spectrum recover
to the initial state. This measurement declares that M-c also has
base-responsive reversibility. The pictures of acid/base
stimulated solutions were shown in Figure 1d, with yellow color
in acid-form M−a−o, colorless in neutral state M−c and red color
in base-form M−b−o.
According to the literature, this heterocyclic ring structure will be
changed to open form after stimulated by the acid.[43] The
structure of the open form is speculated as M−a−o (Figure 1e).
M−r−1 with methoxyl exhibits the similar acid responsive
property with M−c as shown in Figure S1, which indicates that
the acid responsive behavior of M−c is dependent on the
heterocyclic ring, not hydroxyl. Refer to the absorption peak at
424 nm of M−r−2 (Figure S2) and the absorption peak at 306 nm
for p-nitrophenol (Figure S3), the emerged absorption peak at
436 nm in Figure 1b is supposed to come from the conjugated
part of the M−a−o, which is same with the structure of M−r−2.
And the absorption peak at 305 nm can be assigned as the
absorption of nitrophenol group. 12 nm red shifts between the
absorption peaks of M−c−o and M−r−2 may be due to the spatial
Figure 2. Partial 1H NMR spectra of M−c (top), M−c with CF3COOD (M−a−o,
middle) and M−c with K2CO3 (M−b−o, bottom) in CD3OD solvent.
NMR measurement to confirm the structures. To
demonstrate our conjecture about the structure of three states of
M−c molecule (M−c, M−a−o, M−b−o), 1H NMR spectroscopy
measurement was carried out, which is the most powerful
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method to get the structure information of the organic molecule.
DFT calculation to speculate switching mechanism. The
particularity of M−c molecule is that this molecule has three
independent states. The switching mechanism of M−c was
speculated as shown in Scheme 1. In the neutral condition, the
stable form for M−c molecule is close form, and C−O bond of
oxazine may split to obtain the open form M−o. When acid was
added, M−c molecule captures proton to form a close form
M−a−c firstly, then C−O bond was broken immediately to switch
to the open-form M−a−o along with color changing from
colorless to yellow. And the proton on the hydroxyl of M−c
molecule could be taken away by base to generate a close form
M−b−c. Then M−b−c would switch to open-form structure
M−b−o with color changing from colorless to red.
DFT calculation method was used to study the mechanism of
color changing. The results are shown in Scheme 1. There is
only one transition state (TS) in the ring-opening reaction of M−c,
in which the bond length of C−O is 2.15 Å. The Gibbs free
energy of the ring-opening reaction is about 11.2 kcal/mol which
indicates the ring-open process from M−c to M−o is unfavorable
in thermodynamic. And activation energy (14.9 kcal/mol is bigger
than 10 kcal/mol) also support that the reaction cannot
spontaneously take place at room temperature. While for the
close-ring process from M−o to M−c, the reaction is great
favorable according to the Gibbs free energy (−11.2 kcal/mol)
and activation energy (3.7 kcal/mol). The calculated results
demonstrate that M−c is more stable than M−o without the
environment of acid or base. These agree with the experimental
results that the close form M−c is stable state compared by the
open form M−o, and once M−o, generated from M−a−o and
M−b−o, will switch to close form (M−c) rapidly.
However, after being stimulated by acid and base, the switching
direction is converted and the open form is stable state. For the
ring-opening process of M−a−c, the bond length of C−O (1.90 Å)
of its transition state (TS−a) is shorter than the ring-opening
process of M−c. And the open-ring process is much easier for
the Gibbs free energy is −5.4 kcal/mol and activation energy is
5.9 kcal/mol. This reaction is unidirectional because the
activation energy is 11.3 kcal/mol for the reverse reaction. Which
indicates the M−a−o is more stable than M−a−c. For base
condition, the transition state (TS−b) has the shortest C−O
length (1.77 Å) among these three ring-opening process. The
reaction from M−b−c to M−b−o is favorable in thermodynamic
according to the Gibbs free energy (−12.7 kcal/mol) and
activation energy (3.6 kcal/mol), while the reverse reaction is
unfavorable in room temperature according to the activation
energy (16.3 kcal/mol). This also indicates the M−b−o is more
stable than M−b−c.
1
The H NMR spectra of M−c, M−c with CF3COOD (M−a−o) and
M−c with K2CO3 (M−b−o), respectively were obtained in CD3OD
solvent as shown in Figure 2. Compared with 1H NMR spectrum
of M−c (black curve), the H NMR spectrum after adding acid
1
(blue curve) exhibits obviously downfield shifts overall. The
downfield shifts of H4−H11 ascribe to the electron-withdrawing
effect of newly generated ammonium cationic. Hereinto, H10
exhibits the largest shift about 1.95 ppm because of the
transition of the electron-donating effect of the nitrogen to the
electron-withdrawing effect. Furthermore, the slight downfield
shift of H1, H2 and H3 declared that the electron density of p-
nitrophenol is reduced when a proton is captured. All of these
results demonstrate that the M−a−o structure generated after
being stimulated by acid.
As for the 1H NMR spectrum of base-form (M−b−o, green curve),
H1, H2 and H3 exhibit obviously highfield shift compared with
M−c (black curve) and acid-form (M−a−o, blue curve), which
indicates that a high-electron-density p-nitrophenol anion is
generated. In addition, H8, H9, H10 and H11 exhibit significantly
highfield shift compared with M−a−o (blue curve) because the
oxygen anion has much stronger electron donor ability.
Compared with M−c (black curve), H8 and H10 exhibit downfield
shift due to the electron-withdrawing effect of ammonium
cationic, while H9 and H11 exhibit highfield shift owing to the
electron-donating effect of oxygen anion. Similarly, H4, H5, H6,
and H7 exhibit downfield shift comparing with M−c (black curve)
due to the electron-withdrawing effect of the adjacent
ammonium cationic, and exhibit highfield shift comparing with
M−a−o (blue curve) due to the electron-donating effect of
oxygen anion on the conjugated structure. All of these
phenomena declare that the M−b−o structure generates after
being stimulated by the base.
Thus, after being stimulated by acid, M−c is switched to M−a−o
through M−a−c. Then with being neutralized by base, M−a−o is
switched back to M−c through M−o. For being stimulated by
base first, and then being neutralized by acid, M−c is switched to
M−b−o through M−b−c first, and then M−b−o switches to M−c
through M−o. Therefore, the switching under the stimulation of
acid and base is unidirectional.
Electrochromic phenomenon and mechanism. The
electrochemical properties of M−c were studied through cyclic
voltammetry. As shown in Figure 3, the first reductive peak of
M−c is at −1.0 V. According to LUMO of M−c calculated by DFT
as shown in Figure S6, this peak is supposed to come from the
reduction of nitro group of the p-nitrophenol part due to its strong
electron-withdrawing property. This speculation is further
Scheme 1. (a) The switching mechanism of M−c molecule responding to acid
and base and the calculated Gibbs Free Energy (ΔG) and Activation Energy
(Ea) of different open-ring processes. The calculated energy of the process
from (b) M−a−c to M−a−o, (c) M−c to M−o, (d) M−b−c to M−b−o and the
structure of transition states.
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confirmed by the similar reductive peak of M−c and p-
nitrophenol in Figure 3. The slight difference of reduction peak
may come from the difference of C−O and H−O between M−c
and p-nitrophenol, and also may come from the spatial electron
density influence of the indole part. The reduction state of nitro
has alkalinity,45,46 which may be used to control the M−c base
sensitive color change, as we expected.
Considering the phenomena and analysis mentioned above, the
electro-base mechanism was shown in Scheme 2. Firstly, the
M−c molecule was reduced to generate M−r−c with an oxygen
anion. The oxygen anion has strong alkalinity which would
capture a proton from another M−c molecule to produce the
M−r−p, switching another M−c molecule to M−b−o with obvious
red color emerging. When the scan direction is reversed, M−r−p
is oxidized to M−c−p form. The active proton on nitro of M−c−p
go back to the M−b−o, switching the M−b−o to close form M−c.
After releasing the proton, the M−c−p turned into the molecule
M−c finishing the electro-base cycle. This reversible color
changing of switchable molecule base on electro-base
mechanism is promising for development a new kind of ECM.
Figure 3. Cyclic voltammograms of M−c (1.0×10−3 M) and p-nitrophenol
(1.0×10−3 M) in acetonitrile with 0.1 M TBAPF6 at scan rate 100 mV/s.
To explore if the M−c molecule has the electro-switchable
properties, the changes of absorption spectra during reductive
processes were recorded with applying potentials in situ as
shown in Figure 4a. Before the voltage being applied, the
absorption peak is at 267 nm, and there is no absorption in the
visible region. After applying a bias voltage (−1.2 V) for 10 s, the
absorption peaks at 545 nm, 513 nm and 432 nm emerge and
increase with the color changing from colorless to red (Figure 4a,
inside photograph). The emerging peaks positions are
absolutely same with that of M−b−o obtained by the t-BuOK,
which demonstrates that the M−b−o structure can also be
obtained by electrical field and M−c molecule exhibits the
electro-base property in reductive process. More importantly, the
electrochromic behavior of the M−c molecule is reversible. The
change of the absorption at 545 nm as the maximum absorption
wavelength of M−b−o was monitored in situ by UV−vis
spectrometers combined with CV as shown in Figure 4b. Before
the voltage is applied, there is no absorption at 545 nm. When
the M−c began to be reduced at −0.85 V, the absorption at 545
nm began to increase. With the voltage absolute value
increasing, the absorption keeps increasing. After the reduced
product of M−c began to be oxidized at −0.9 V, the absorption
began to decrease until to zero.
Scheme 2. The mechanism of electrochromic properties of M−c molecule
through combining its electro-base property and acid/base sensitive properties.
Figure 5. (a) The model of electrochromic device structure. (b) The UV−vis
spectra of electrochomic device; (c) Square-wave potential step
absorptiometry of electrochromic device (cycle switching time: −1.54 V 600 ms,
cut off 0.5 s, then 1 V 4 s, cut off 1 s) and (d) The color efficiency of the
electrochromic device. The b, c and d was recorded at 545 nm.
Electrochromic device. To extend the application of the three-
states switchable molecules, an electrochromic device was
fabricated. The model of this electrochromic device structure
was shown in Figure 5a. A thin electrochromic layer and an ion
Figure 4. (a) The electrochemical spectra of M−c (1.0×10−4 M with 0.1 M
TBAPF6 in acetonitrile) under no stimulation and bias voltage at −1.2 V for 10 s
measured in situ. (b) Absorption of M−c (1.0 × 10−4 M with 0.1 M TBAPF6 in
acetonitrile) changes (top) with the cyclic voltage being applied.
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Acetonitrile
(chromatographic
grade),
tetrabutylammonium
storage layer were separated by an ion conductive layer. The
three functional layers were sandwiched between two layers of
transparent conducting ITO glasses. The ion conductive layer
was used to transport the electrolyte ion and prevent the M−c
diffuse to counter layer. In order to balance the charge in the
device, ion storage layer containing hydroquinone and
benzoquinone was used.
hexafluorophosphate (TBAPF6), trifluoroacetic acid (TFA) and potassium
tert-butoxide (t-BuOK) were purchased from Aladdin Reagent Company
(Shanghai, China). Polymethyl methacrylate (PMMA), propylene
carbonate (PC), p-benzoquinone, p-hydroquinone, p-nitrophenol, p-
anisaldehyde and p-hydroxybenzaldehyde were purchased from Energy
Chemical Company (Shanghai, China). TBAPF6 was recrystallized three
times from ethanol and dried under vacuum at 80°C prior to use. Other
chemicals (analytical grade) were used as received without further
purification.
The absorption spectra of the device was recorded by applying a
potential in suit in Figure 5b. Before being stimulated by bias
voltage, the device shows yellow color. After the −1.0 V voltage
being applied on the device, the broad absorption peaks at 545
nm, 513 nm appeared with the color changing from yellow to red
(blue curve). It is evident that the molecule M−c was switched to
M−b−o by electro-base. When a positive voltage (+1.0 V) is
applied, the color of the device returns to yellow (red curve) and
the peak at 545 nm, 513 nm disappears. The beautiful red text
on the device patterned by laser etching method can display
reversible stimulated by −1.2 V/1.0 V as shown in the inset
picture of Figure 5b. This device is very sensitive to electrical
field. The absorption change can be detected by spectrometer
even if the stimulating time is as short as 1 millisecond in Figure
S7. Furthermore, the reversibility of this device was also
monitored. As shown in Figure 5c, no decay of absorption can
be seen after 2000 switching cycles. It reveals that this
electrochromic device has excellent reversibility and highly
durability. In addition, coloration efficiency is an important index
to evaluate the electron efficiency of electrochromic device.
Herein, coloration efficiency was estimated to be as high as 804
cm2/C, which is extremely high compared with traditional small
molecule and metal oxide electrochromic device (Figure 5d).
These measurements declare that the multifunctional M−c
molecule can be used to fabricate high performance
electrochromic devices.
Preparation of Electrochromic device
First, a solution for fabricating electrochromic layer was prepared by
dissolving 0.36 g of PMMA, 0.075 ml of PC, 0.15 g of TBAPF6 and 8.29
mg of M−c molecules in 2 ml of acetonitrile. A solution for fabricating ion
storage layer was prepared by dissolving 0.9 g of PMMA, 0.187 ml of PC,
0.375
g of TBAPF6, 54 mg of p-benzoquinone and 110 mg of
hydroquinone in 10 ml of acetonitrile. A solution to make conductive layer
was prepared by dissolving 3.6 g of PMMA, 0.75 ml of PC and 1.5 g of
TBAPF6 in 20 ml of acetonitrile. The mixtures were stirred for 5 h at 40 °C.
Three homogeneous and transparent solutions were obtained. The
electrochromic layer and the ion storage layer were respectively
dropcasted on two ITO glasses. After drying at room temperature for the
ion storage layer, a layer of conductive layer was dropcasted on the top.
After the solvent was evaporated, the two ITO glasses were clamped
together that the following a transparent electrochromic device with
symmetrically layered was obtained.
Synthesis of M−c and M−r−1 and M−r−2
M−c was prepared through the following procedures[48]: 5a,6,6-trimethyl-
2-nitro-5a,6-dihydro-12H-benzo[5,6][1,3]oxazino[3,2-a]indole (SM1) (1.6
mmol, 496 mg) and p-hydroxybenzaldehyde (2.2 mmol, 268 mg) were
heated and refluxed in 25 ml ethanol solution for 8 h under argon
atmosphere. After cooling to ambient temperature, the solvent was
removed with rotary vacuum evaporator and treated with NaHCO3
aqueous solution. Acetic ether was added to extract the product and
dried with anhydrous sodium sulfate. The solvent was distilled off under
reduced pressure. The residual solid was purified by flash
chromatography (petroleum ether/ethyl acetate = 5:1) to obtain a red
solid powder(412 mg, 62%). 1H NMR (500 MHz, DMSO) δ = 9.63 (s, 1H),
8.13 (s, 1H), 8.02 – 7.91 (m, 1H), 7.36 (d, J = 8.2 Hz, 2H), 7.18 (d, J =
7.1 Hz, 1H), 7.07 (t, 1H), 6.96 (d, J = 9.1 Hz, 1H), 6.84 (d, J = 7.8 Hz, 1H),
6.81 (t, 1H), 6.72 (d, J = 8.1 Hz, 3H), 6.30 (d, J = 16.1 Hz, 1H), 4.74 (s,
2H), 1.32 (s, 6H). 13C NMR (126 MHz, DMSO) δ = 159.55, 158.46,
147.03, 140.35, 138.32, 135.74, 128.97, 127.94, 126.89, 124.20, 124.08,
122.67, 121.28, 120.75, 118.03, 115.94, 109.48, 104.77, 49.92, 49.07,
23.57, 22.50. LC-HRMS: m/z [M+H]+ calculated for 415.1652, found
415.1649. Melting point: 153.1−153.9 °C.
Conclusions
A multi-functional molecular switch M−c with excellent acid and
base sensitive properties was designed and synthesized.
Structures of its respective three states, neutral-form M−c, acid-
form M−a−o and base-form M−b−o, were speculated by
comparing the absorption spectra with reference molecules and
proved by NMR. The structure changing mechanism of M−c
under the stimulation of acid/base was analyzed by DFT
calculation. This multi-function switchable molecule can be used
not only as an acid/base indicator, but as electro-base. An
electrochromic device based on electro-base and acid/base
responsive properties was successfully fabricated. Color
switching between red and yellow can be controlled by altering
voltages. The device exhibits good properties with good
reversibility (2000 cycles) and high coloration efficiency (804
cm2/C). This successful attempt for using molecular switches as
electrochromic material provides a novel thoughts for the
creation of new electrochromic devices and other display
devices.
M−r−1 was synthesized through the following procedures : 5a,6,6-
trimethyl-2-nitro-5a,6-dihydro-12H-benzo[5,6][1,3]oxazino[3,2-a]indole
(SM1) (1 mmol, 310 mg), p-anisaldehyde (2 mmol, 272 mg) and
methanesulfonic acid (1.5 mmol, 144 mg) was heated and refluxed at 10
ml ethanol solution for 4 h under argon atmosphere. After cooling to
ambient temperature, the solvent was removed with rotary vacuum
evaporator and treated with NaHCO3 aqueous solution. Acetic ether was
added to extract the product and dried with anhydrous sodium sulfate.
The solvent was distilled off under reduced pressure. The crude product
was precipitate through adding n-hexane to acetic ether. And the crude
product was further purified by flash chromatography (petroleum
ether/ethyl acetate = 3:1 with 0.5 % volume fraction of triethylamine) to
obtain a white solid powder (351 mg, 82%). 1H NMR (500 MHz, CDCl3) δ
8.00 (s, 1H), 7.97 (d, J = 9.0 Hz, 1H), 7.34 (d, J = 8.5 Hz, 2H), 7.16 –
7.07 (m, 2H), 6.86 (d, J = 8.7 Hz, 4H), 6.74 (d, J = 16.1 Hz, 1H), 6.61 (d,
J = 7.8 Hz, 1H), 6.20 (d, J = 16.1 Hz, 1H), 4.58 (s, 2H), 3.80 (s, 3H), 1.37
(s, 6H). 13C NMR (126 MHz, CDCl3) δ 160.10, 159.42, 146.58, 140.70,
138.36, 135.65, 128.45, 128.26, 127.73, 124.07, 123.33, 122.42, 121.54,
120.77, 120.15, 117.75, 114.24, 108.86, 104.02, 55.47, 50.12, 40.79,
Experimental Section
Materials
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26.61, 18.73. LC-HRMS: m/z calculated for C26H24N2O4 429.1809, found
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
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Multifunctional molecular switch:
By endowing the electro-base
New-type Electrochromic Material
Xiaojun Wang, Chang Gu, Hongzhi
Zheng, Yu-Mo Zhang,* Sean Xiao-An
Zhang*
property and acid/base responsive
properties with oxazine molecular
switch, a new kind of electrochromic
material with great performance was
successfully fabricated.
Page No. – Page No.
A multi-stimuli-responsive oxazine
molecular switch: A new strategy for
the design of electrochromic
materials
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