Multistimuli-responsive small-molecule compound with aggregation-induced emission enhancement characteristics: preparation, properties and applications

Article abstract of DOI:10.1039/c8nj04962a

A novel multifunction luminescent small-molecule compound with hydrophilic chains was successfully designed and synthesized, which exhibited aggregation-induced emission enhancement (AIEE) characteristics. Even more interesting, it exhibited responsive be

Full text of DOI:10.1039/c8nj04962a

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Multistimuli-responsive small-molecule
compound with aggregation-induced emission
enhancement characteristics: preparation,
properties and applications
Cite this: New J. Chem., 2018,
42, 20239
Qiangjun Li, Yongjie Yuan, Lifang He, Shenglan Liu and Hailiang Zhang
*
A novel multifunction luminescent small-molecule compound with hydrophilic chains was successfully
designed and synthesized, which exhibited aggregation-induced emission enhancement (AIEE)
characteristics. Even more interesting, it exhibited responsive behavior to temperature, pH and UV
irradiation. In aqueous solution, its luminescence intensity showed temperature sensitivity. Initially, its
luminescence intensity decreased slightly with an increase in temperature, which was enhanced greatly
when the temperature increased to its lower critical solution temperature (LCST). With a further increase
in temperature, there was a significant decline in its luminescence intensity. Additionally, it showed
different pH and UV irradiation responsive behaviors in water and THF solution. In THF solution, its
luminescence intensity was enhanced greatly with the addition of alkali, but it decreased with an
increase in UV irradiation time. In aqueous solution, its luminescence intensity was enhanced slightly and
its LCST decreased first and then increased with the addition of alkali. However, its luminescence
intensity increased with an increase in UV irradiation time. The prepared small-molecule compound
could also be used as a rewritable material with solid alkali or alkali solution used as ink and ordinary
paper adsorbed with our prepared small-molecule compound used as rewritable paper.
Received 30th September 2018,
Accepted 10th November 2018
DOI: 10.1039/c8nj04962a
rsc.li/njc
materials with dual or multistimuli-responsive properties is still a
big challenging due to the difficulties of combining different
1. Introduction
In the past years, stimuli-responsive luminescent materials have stimuli blocks within one molecule. To date, most of the reported
gained considerable attention due to their ‘‘smart’’ behavior.1–5 stimuli-responsive luminescent materials only exhibited single
Compared with traditional luminescent materials, the luminescence stimulus responsive behavior, and only a few reported luminescent
color and intensity of stimuli-responsive luminescent materials materials displayed dual or multistimuli-responsive properties.27–29
change greatly with the application of external stimuli.6–11 This Zhu and co-workers reported a novel boron-difluoride complex,
unique property endows them with wide applications in mechan- which was sensitive to mechanical grinding, organic solvent
ochromic materials,12–15 temperature sensors,16–20 pH detectors,21–23 vapours and acid/base vapours.30 Chao and co-workers prepared
pressure sensors,24,25 anti-counterfeiting materials,26 etc. A large a novel polyamic acid, which was sensitive to redox species, pH,
number of stimuli-responsive luminescent materials have been electrochemical, and pressure stimuli.31 Xu and co-workers
reported and studied extensively. According to the types of prepared a novel multistimuli-responsive molecule based on
stimuli, stimuli-responsive luminescent materials can be divided tetraphenylethene.32 The tetraphenylethene derivatives were
into temperature stimuli-responsive luminescent materials, pH responsive to grinding, heating, solvent-fuming, and acid and
stimuli-responsive luminescent materials, mechanical stress base vapors stimuli. Although some fluorescent materials with
stimuli-responsive luminescent materials, light stimuli-responsive multistimuli-responsive properties have been prepared, reports
luminescent materials, etc.
on dual or multistimuli-responsive luminescent materials are
The growing demand for smart materials has promoted the still rare owing to the above mentioned problems.
development of luminescent materials with dual or multistimuli-
In 2002, Soo Young Park found for the first time that an
responsive behavior. However, the preparation of luminescent a-cyanostilbenic derivative exhibited AIEE characteristics.33
The synthetic a-cyanostilbenic derivative 1-cyano-trans-1,2-bis-
(40-methylbiphenyl)-ethylene (CN-MBE) emits weak fluorescence
Key Laboratory of Polymeric Materials and Application Technology of Hunan
Province, Key Laboratory of Advanced Functional Polymer Materials of Colleges and
in dilute solution, but its emission intensity is enhanced greatly
in the aggregate state. Subsequent studies also suggested that
Universities of Hunan Province, College of Chemistry, Xiangtan University,
Xiangtan 411105, Hunan Province, China. E-mail: hailiangzhang@xtu.edu.cn
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a-cyanostilbenic derivatives show responsive behavior to a PTIQm40 luminescence spectrometer equipped with a tem-
UV irradiation.34,35 With an increase in UV irradiation time, perature regulating device. The UV irradiation test was carried
these molecules change from the trans-conformation to cis- out with a 365 nm ultraviolet lamp (1000 MW). A Zetasizer Nano
conformation and their luminescence intensity is reduced gradually. ZS90 particle size analyzer was applied to measure the size
Therefore, a-cyanostilbenic derivatives can be used as light stimuli- distribution of the HPTEO3PA aqueous solution and HPTEO3PA
responsive luminescent materials. After chemical modification, THF solution.
a-cyanostilbenic derivatives can also exhibit temperature stimuli-
responsive behavior. Soo Young Park reported two different
2.3 Synthesis of multistimuli-responsive small-molecule
compound
amphiphilic molecules by incorporating oligo(ethylene glycol)
units into dicyanodistyrylbenzene.36 Both molecules exhibited
The preparation routes for the multistimuli-responsive small-
temperature stimuli-responsive behavior and their luminescence
wavelength varied with temperature. However, there are no
reports on dual or multistimuli-responsive luminescent materials
based on a-cyanostilbenic.
molecule compound HPTEO3PA are shown in Fig. 1. The
specific preparation method for HPTEO3PA is as follows:
Synthesis of 2-(2-(2-methoxyethoxy)ethoxy)ethyl 4-methyl-
benzenesulfonate. Triethylene glycol monomethyl ether (30.0 g,
0.183 mol), 5% sodium hydroxide solution (9.96 g, 0.249 mol)
and 100 mL THF were added to a 500 mL round-bottom flask.
Then the round-bottom flask was placed in an ice bath. 4-Toluene
sulfonyl chloride (31.66 g, 0.166 mol) was dissolved in 50 mL THF
and then added dropwise to the round-bottom flask. When the
THF solution was completely added to the round bottom flask, the
reaction mixture was continued with stirring at room temperature
for 3 h. Subsequently, the desired product (48.06 g, yield: 82.5%)
was obtained through extraction with H2O and CH2Cl2, drying
with anhydrous sodium sulfate and desolventizing. 1HNMR
(400 MHz, CDCl3, d): 7.76 (d, 2H, Ph–H), 7.32 (d, 2H, Ph–H),
4.18–4.15 (m, 2H, –SO3CH2–), 3.70–3.66 (m, 2H, –OCH2–), 3.62–
3.58 (m, 6H, –OCH2–), 3.56–3.50 (m, 2H, –OCH2–), 3.34 (s, 3H,
–OCH3), 2.42 (s, 3H, Ph–CH3). Mass spectrometry (MS) (m/z)
calcd for C14H22O6S, 318.11; found, 319.10.
Synthesis of methyl 3,4,5-tris(2-(2-(2-methoxyethoxy)ethoxy)-
ethoxy)benzoate. Methyl gallate (5.12 g, 0.0278 mol), anhydrous
potassium carbonate (23.11 g, 0.167 mol) and 200 mL 1,4-
dioxane were added to a 500 mL round-bottom flask and the
mixture were stirred at room temperature for 1 h. Subsequently,
2-(2-(2-methoxyethoxy)ethoxy)ethyl-4-methylbenzenesulfonate
(40.0 g, 0.126 mol) was added into the mixture and then the
reaction mixture was heated at 110 1C for 36 h. Afterward,
the mixture was treated by filtration and the organic phase
was collected and the solvent removed with a rotary evaporator
to obtain the crude product. The crude product was further
purified by column chromatography with ethyl acetate : acetone
Herein, we report a novel multistimuli-responsive luminescent
material based on a-cyanostilbenic. To achieve this aim, (Z)-2-(4-
hydroxyphenyl)-3-(3,4,5-tris(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-
phenyl)acrylonitrile (HPTEO3PA) was synthesized (Scheme 1).
To endow the molecule with pH stimuli-responsive properties,
the –OH group was introduced, and for temperature stimuli-
responsive properties, oligo(ethylene glycol) blocks were introduced.
Since a-cyanostilbenic derivatives have their own properties as well
as AIEE properties, the synthetic molecule was expected to
exhibit AIEE characteristics and show temperature and pH
stimuli-responsive properties. The application of our prepared
dual-stimuli luminescent compound in temperature sensors,
alkali detectors and rewritable materials was also explored.
2. Experimental section
2.1 Materials
Triethylene glycol monomethyl ether, 4-toluene sulfonyl chloride,
methyl gallate, lithium aluminum hydride, manganese dioxide,
potassium tert-butylate, and 4-hydroxybenzyl cyanide were purchased
from energy chemical. Tetrahydrofuran, ethanol, acetone, ethyl
acetate and other solvents were used as received without further
purification.
2.2 Measurements and characterizations
1H NMR analysis of HPTEO3PA was performed on a Bruker
ARX400 MHz spectrometer with tetramethylsilane (TMS) as an
internal standard. The molecular weight of the synthetic molecules
was measured using a Bruker Biflex III MALDI-TOF spectrometer.
The absorption spectra of HPTEO3PA and its lower critical solution
temperature (LCST) were recorded on an Agilent Carry 100
spectrophotometer equipped with a temperature regulating
device. Fluorescence spectra of HPTEO3PA were recorded on
Fig. 1 Synthetic routes for the multistimuli-responsive small-molecule
compound HPTEO3PA.
Scheme 1 Chemical structure of HPTEO3PA.
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(3 : 1, v/v) as the eluent to give the final product (13.21 g, yield: 3. Results and discussion
1
80%) as a yellowish liquid. H NMR (d, ppm, CDCl3): 7.95 (s,
3.1 Chemical structure characterization of multistimuli-
2H, Ph–H); 4.31–4.27 (t, 6H, –OCH2–); 3.88 (s, 3H, –COOCH3);
3.86–3.80 (t, 6H, –OCH2–); 3.74–3.72 (t, 6H, –OCH2–); 3.67–3.63
(t, 12H, –OCH2–); 3.55–3.53 (t, 6H, –OCH2–); 3.37 (s, 9H,
–OCH3). Mass spectrometry (MS) (m/z) calcd for C29H50O14,
622.32; found, 623.30.
responsive luminescent molecule HPTEO3PA
The multistimuli-responsive luminescent molecule HPTEO3PA
was successfully synthesized via the synthetic routes illustrated
1
in Fig. 1. H NMR and mass spectroscopy were used to inves-
tigate the chemical structure of HPTEO3PA. Fig. 2a shows the
1H NMR spectrum of HPTEO3PA. As observed, all the peaks in
the 1H NMR spectrum correspond to the hydrogen atoms in the
molecule and the integral area of the peaks was in good
agreement with the number of hydrogen atoms. This indicated
that we obtained pure HPTEO3PA. The mass spectrum further
confirmed the chemical structure of HPTEO3PA. As seen from
Fig. 2b, the measured molecular weight was 708.332, 709.350
and 710.372, which was in good agreement with the theoretical
molecular weight of HPTEO3PA of 707.35, 708.36 and 709.36,
respectively.
Synthesis of (3,4,5-tris(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-
phenyl)methanol. Aluminum hydride (0.07 g, 0.01748 mol), and
50 mL THF were added to a 250 mL round-bottom flask. Methyl
3,4,5-tris(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)benzoate (7.4 g,
0.0119 mol) was dissolved in 40 mL THF and then dropped
into the round-bottom flask. Afterward, the reaction mixture
was stirred at room temperature for 3 h. When the reaction was
completed, dilute hydrochloric acid was added to the above
mixture and a clear and transparent solution was obtained.
Subsequently, the desired product (6.61 g, yield: 93.5%) was
obtained through extraction with H2O and CH2Cl2, drying with
anhydrous sodium sulfate and desolventizing. Yield: 93.5%.
1H NMR (d, ppm, CDCl3): 6.62 (s, 2H, Ph–H); 4.58 (s, 2H, Ph–
CH2–O); 4.27–4.00 (t, 6H, –OCH2–); 3.85–3.76 (t, 6H, –OCH2–);
3.73–3.71 (t, 6H, –OCH2–), 3.67–3.63 (t, 12H, –OCH2–); 3.56–3.53
(t, 6H, –OCH2–); 3.38 (s, 9H, –OCH3). Mass spectrometry (MS)
(m/z) calcd for C28H50O13, 594.33; found, 595.32.
Synthesis of 2,3,4-tris(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-
benzaldehyde. A mixture of (3,4,5-tris(2-(2-(2-methoxyethoxy)ethoxy)-
ethoxy)phenyl)methanol (6.0 g, 0.01 mol), MnO2 (17.55 g, 0.202 mol)
and 120 mL dry dichloromethane was added to a 250 mL round-
bottom flask. The reaction mixture was stirred for 12 h at room
temperature. After the reaction was finished, the mixture was
treated by filtration and the organic solvent was removed with a
rotary evaporator to the obtain crude product, which was further
purified by column chromatography with ethyl acetate : acetone
(3 : 1, v/v) as the eluent to afford a yellowish liquid product
(5.45 g, yield: 92%). 1H NMR (d, ppm, CDCl3): 6.96 (s, 2H, Ph–H);
4.37–4.27 (t, 6H, –OCH2–); 3.84–3.75 (t, 6H, –OCH2–); 3.72–3.69
(t, 6H, –OCH2–), 3.65–3.62 (t, 12H, –OCH2–); 3.58–3.55 (t, 6H,
–OCH2–); 3.37 (s, 9H, –OCH3). Mass spectrometry (MS) (m/z)
calcd for C28H48O13, 592.31; found, 593.30.
3.2 AIEE properties of HPTEO3PA
We also investigated the photophysical properties of HPTEO3PA.
Fig. 3a shows the UV spectrum of the HPTEO3PA THF solution.
As observed, HPTEO3PA exhibited three obvious absorption
peaks at about 202 nm, 217 nm and 343 nm. The fluorescence
luminescence behavior of HPTEO3PA was also studied and the
experimental results indicated that HPTEO3PA also possessed
AIEE characteristics. Fig. 3b shows the fluorescence spectra of
HPTEO3PA in THF/petroleum ether mixtures. As observed,
HPTEO3PA emitted weak fluorescence in pure THF solution.
With the addition of non-solvent petroleum ether, the fluores-
cence intensity remained constant at first and then increased
greatly. The maximum luminescence wavelength also remained
unchanged and exhibited an obvious red shift when the volume
fraction of petroleum ether increased to 95%. The UV absorption
spectra changes of HPTEO3PA depending on the petroleum ether
fraction in THF were measured. As observed from Fig. 3c, when
the volume fraction of petroleum ether was lower than 90%, its
UV absorption spectrum was almost unchanged with an increase
in petroleum ether. When the volume fraction of petroleum ether
was higher than 90%, a new UV absorption band in the range of
400 nm to 500 nm was observed and its intensity increased with
an increase in the volume fraction of petroleum ether. The new
UV absorption band should be ascribed to the formation of
J-aggregates. The new generation of an emission peak at about
480 nm can also be explained by the formation of J-aggregates.
Synthesis of (Z)-2-(4-hydroxyphenyl)-3-(3,4,5-tris(2-(2-(2-meth-
oxyethoxy)ethoxy)ethoxy)phenyl)acrylonitrile (HPTEO3PA).
A
mixture of 2,3,4-tris(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)benz-
aldehyde (5.0 g, 0.0084 mol), 4-hydroxybenzyl cyanide (1.35 g,
0.010 mol), potassium tert-butylate (1.89 g, 0.0169 mol) and
100 mL anhydrous ethanol were added to a 250 mL round-
bottom flask. The reaction mixture was heated at 80 1C for 3 h.
Subsequently, the crude product was obtained through extraction
with H2O and CH2Cl2, drying with anhydrous sodium sulfate and
desolventizing. The crude product was further purified by column
chromatography with ethyl acetate : acetone (3 : 1, v/v) as the eluent
to obtain the desired product (3.88 g, yield: 65%). 1H NMR (d, ppm,
CDCl3): 7.39–7.37 (d, 2H, Ph–H), 7.16 (s, 1H,QCH–), 7.03 (s, 2H,
Ph–H), 6.83–6.81 (d, 2H, Ph–H), 4.24–4.14 (t, 6H, –OCH2–), 3.88–
3.82 (t, 6H, –CH2O–), 3.77–3.36 (t, 24H, –OCH2–, –CH2O–),
3.369 (s, 9H, –OCH3). Mass spectrometry (MS) (m/z) calcd for
C36H53NO13, 707.35; found, 708.33.
Fig. 2 1H NMR spectrum (a) and MS spectrum (b) of HPTEO3PA in CDCl3.
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Fig. 4 Photographs illustrating the transmittance change in the HPTEO3PA
aqueous solution.
Fig. 3 (a) UV spectrum of HPTEO3PA THF solution, (b) fluorescence
spectra and (c) UV spectra of HPTEO3PA in THF/petroleum ether mixtures
with different petroleum ether fractions (fp). Excitation wavelength:
365 nm and concentration = 5 ꢁ 10ꢀ5 mol Lꢀ1
.
The UV spectra and fluorescence spectra of HPTEO3PA in THF/
petroleum ether mixtures indicated that HPTEO3PA possessed
AIEE characteristics and its enhanced fluorescence intensity
and obvious red shift can be explained by the formation of
J-aggregates and the planarization of the HPTEO3PA molecules.33
3.3 Temperature stimuli-responsive properties and
applications of HPTEO3PA
Fig. 5 (a) Temperature–transmittance curves of HPTEO3PA aqueous
solution, (b) LCST–concentration curves of HPTEO3PA aqueous solution
and (c) transmittance curves of HPTEO3PA (5 mg mLꢀ1) upon heating and
cooling. Measured wavelength: 600 nm.
The temperature stimuli-responsive properties of an HPTEO3PA
aqueous solution was investigated in detail. The experimental
results showed that HPTEO3PA with longer hydrophilic chain
was soluble in water at a lower temperature, while insoluble in aqueous solution has a great influence on its thermosensitivity.
water at higher temperature, which indicated that HPTEO3PA The LCST of the HPTEO3PA aqueous solution decreased with an
exhibited temperature stimuli-responsive properties. For example, increase in concentration (Fig. 5b). As observed from Fig. 5c, the
HPTEO3PA could dissolve in water to form a transparent solution transmittance curves of the HPTEO3PA aqueous solution
(5 mg mLꢀ1) at 25 1C. However, the transparent solution became (5 mg mLꢀ1) on heating and cooling basically overlapped,
an opaque turbid fluid when the temperature increased to 50 1C, suggesting that the thermosensitivity of the HPTEO3PA aqueous
which is higher than the lower critical solution temperature solution showed good reversibility.
(LCST) of HPTEO3PA. The specific transformation process is
As mentioned above, when the temperature increased to the
shown in Fig. 4.
LCST, HPTEO3PA aggregated. Meanwhile, HPTEO3PA is AIEE
To further investigate the thermosensitivity of HPTEO3PA, molecule. Therefore, the aggregation of HPTEO3PA caused by
an Agilent Carry 100 spectrophotometer equipped with a tempera- heating will improve its luminescence intensity. Specifically,
ture regulating device was applied to measure the transmittance the luminescence intensity will increase with an increase in
of HPTEO3PA aqueous solutions with different concentrations temperature. To confirm this conjecture, we measured the
on heating and cooling. The heating and cooling rate was luminescence intensity of an HPTEO3PA aqueous solution
1.0 1C minꢀ1. As observed from Fig. 5a, the transmittance of (7.5 mg mLꢀ1) at different temperatures. As seen from Fig. 6a,
the HPTEO3PA aqueous solution indicated obvious thermo- the change in its luminescence intensity can be divided into
sensitivity. At lower temperature, HPTEO3PA was soluble in water, three stages. Initially, the temperature was lower than LCST,
which resulted in a higher transmittance. The transmittance of and the luminescence intensity decreased slightly with an
the HPTEO3PA aqueous solution dramatically decreased when increase in temperature. This is because HPTEO3PA is more
the temperature increased above its LCST, and the transparent likely to dissipate energy through non-radiation transitions with
aqueous solution turned into an opaque turbid fluid due to the increasing temperature. In the second stage, the temperature
aggregation of HPTEO3PA. The concentration of the HPTEO3PA increased to its LCST, and the luminescence intensity was
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of HPTEO3PA was weakened, resulting in a decline in its LCST.
As the mole fraction of NaOH continued to increase, the –OH
groups reacted with NaOH to yield –ONa, leading to an increase
in the hydrophilicity of the whole molecule. Therefore, the LCST
increased with increasing mole fraction of NaOH. When the mole
ratio of NaOH to HPTEO3PA was greater than 1: 1, all the –OH
groups were translated into –ONa, and the LCST was approxi-
mately independent of the NaOH mole fraction.
We also studied the relationship between pH and the
photophysical properties of the HPTEO3PA aqueous solution.
Fig. 6 (a) Fluorescence spectra of HPTEO3PA aqueous solution (7.5 mg mLꢀ1
measured at different temperatures and (b) plot of I/I25 values versus
)
temperature for HPTEO3PA aqueous solution (7.5 mg mLꢀ1). Excitation Fig. 8a shows the UV spectra of HPTEO3PA aqueous solutions
wavelength: 365 nm.
with different mole fractions of NaOH. As observed, the absorption
peak shifted to a shorter wavelength firstly and then shifted to a
longer wavelength with the addition of NaOH. As mentioned
enhanced greatly due to the aggregation of HPTEO3PA. The
above, the addition of NaOH would weaken the hydrophilicity of
luminescence intensity measured at 49 1C was 39.95-fold that
HPTEO3PA, and therefore HPTEO3PA will aggregate to form a more
measured at 25 1C. In the third stage, further heating led to a
twisted conformation, resulting in a blue-shift in its absorption
slight decrement in emission intensity. This is because the
peak. As the mole fraction of NaOH continue to increase, the
aggregation degree of HPTEO3PA was almost unchanged with
hydrophilicity of HPTEO3PA was enhanced to form a more planar
a further increase in temperature. On the contrary, HPTEO3PA is
conformation, resulting in a red-shift in its absorption peak. Fig. 8b
more likely to dissipate energy through non-radiation transitions
shows the fluorescence spectra of HPTEO3PA aqueous solutions
at higher temperature, leading to a decrement in its emission
with different mole fractions of NaOH. The fluorescence intensity
intensity. The variable temperature fluorescence test indicated
increased due to aggregation.
that the fluorescence luminescence behavior of HPTEO3PA also
In THF solution, the photophysical properties of HPTEO3PA
exhibited temperature stimuli-responsive properties and it can be
changed greatly when alkali was added. Fig. 9(a) shows the
used as a temperature sensor.
normalized fluorescence spectra of HPTEO3PA THF solutions
with addition of different alkalis. As observed, the maximum
luminescence wavelength shifted to a longer wavelength and
3.4 pH stimuli-responsive properties and applications of
HPTEO3PA
The introduction of the –OH group endows HPTEO3PA with pH
stimuli-responsive properties. Thus, we initially studied the
influence of pH on the LCST of an HPTEO3PA aqueous
solution. Fig. 7a shows the temperature–transmittance curves
of HPTEO3PA aqueous solutions (5 mg mLꢀ1) with different
mole fraction of NaOH. As observed, the mole fraction of NaOH
had a great influence on the thermosensitivity of the HPTEO3PA
aqueous solution. Fig. 7b shows the LCST curves of HPTEO3PA
aqueous solution. With an increment in NaOH mole fraction,
the LCST of HPTEO3PA decreased firstly and then increased.
When the mole ratio of NaOH to HPTEO3PA was over 1 : 1, the
LCST remained unchanged even with an enhanced mole fraction
of NaOH. The addition of NaOH destroyed the hydrogen bonding
between water and HPTEO3PA,37 and therefore the hydrophilicity
Fig. 8 UV spectrum (a) and fluorescence spectrum (b) of HPTEO3PA
aqueous solution with different mole fractions of NaOH. Excitation wave-
length: 365 nm and concentration = 5 ꢁ 10ꢀ5 mol Lꢀ1
.
Fig. 9 (a) Normalized fluorescence spectra of HPTEO3PA THF solution
with the addition of different alkalis and (b) fluorescence spectra of HPTEO3PA
THF solution alone and with the addition of octylamine. Excitation wavelength:
365 nm for HPTEO3PA and 440 nm for HPTEO3PA with added alkaline
Fig. 7 (a) Temperature–transmittance curves of HPTEO3PA aqueous
solution (5 mg mLꢀ1) with different mole fractions of NaOH and (b) LCST
curve of HPTEO3PA aqueous solution (5 mg mLꢀ1) with different mole
fractions of NaOH. Measured wavelength: 600 nm.
substance. Concentration = 5 ꢁ 10ꢀ5 mol Lꢀ1
.
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Fig. 10 Pictures of HPTEO3PA THF solution and HPTEO3PA THF solution
with the addition of octylamine under 365 nm UV light. Concentration =
5 ꢁ 10ꢀ5 mol Lꢀ1
.
Fig. 12 Fluorescence spectra of HPTEO3PA in CH2Cl2 and methanol
solutions with the addition of NaOH. Excitation wavelength: 365 nm and
the higher the alkalinity, the greater its red shift. The maximum
luminescence wavelength of the HPTEO3PA THF solution was
429 nm, while it shifted to 537 nm when solid NaOH or solid KOH
was added. Considering this interesting property, HPTEO3PA can
be used as a detector for alkali. The luminescence intensity of
HPTEO3PA THF solution enhanced greatly with the addition of
alkaline substances. The luminescence intensity was enhanced
255-fold when octylamine was added (Fig. 9b). The luminescence
color also changed from blue to yellowish green upon the addition
of octylamine (Fig. 10). The luminescence intensity and the
obvious red-shift should be attributed to the intramolecular
charge transfer effect (ICT). With the addition of alkali, the –OH
groups interacted with the alkali, resulting in oxygen atoms
with a negative charge. Therefore, the electron donating ability
of the oxygen atoms was enhanced greatly. Since –CN is an
electron-withdrawing group, the HPTEO3PA molecules have a
strong ICT effect with the addition of alkali. Meanwhile, the
higher the alkalinity, the stronger the electron-donating ability
of the oxygen atom, resulting in a longer wavelength fluores-
cence emission peak.
concentration (c) = 5 ꢁ 10ꢀ5 mol Lꢀ1
.
unchanged; however, a new absorption peak was observed at
450 nm, which was derived from the intramolecular charge-transfer.
We also measured the fluorescence spectra of HPTEO3PA in CH2Cl2
and methanol solutions with the addition of NaOH. As observed
from Fig. 12, the HPTEO3PA CH2Cl2 solutions showed one obvious
emission at about 408 nm and an obvious emission band ranging
from 510 nm to 610 nm. These two emission peaks powerfully prove
the existence of the ICT process. In methanol solution, HPTEO3PA
only showed one obvious emission at about 520 nm. This is because
strong polar methanol is conducive to the stability of ICT state and
therefore enhanced the ICT process greatly.
We also developed a rewritable material based on HPTEO3PA.
Solid alkali or alkali solution was applied as ink and ordinary
paper adsorbed with HPTEO3PA was used as rewritable paper. As
shown in Fig. 13a, the ordinary paper adsorbed with HPTEO3PA
emitted blue fluorescence under 365 nm UV light. We then wrote
‘‘XTU’’ on this paper using solid NaOH as the ink. The three
letters ‘‘XTU’’ emitted stronger yellow-green fluorescent light
under 365 nm UV light and the other unwritten area still emitted
blue fluorescence, showing stronger visual contrast (Fig. 13b).
Furthermore, the written word could be easily removed. As seen
from Fig. 13c, when the paper was fumigated with hydrochloric
acid, the three letters ‘‘XTU’’ were removed quickly and the whole
paper reverted to its original state, showing blue fluorescence
To further understand the influence of alkali on the photo-
physical properties of HPTEO3PA in different solvents, we measured
the UV spectra of HPTEO3PA aqueous solution, HPTEO3PA
aqueous solution with added NaOH, and HPTEO3PA THF
solution with solid NaOH (Fig. 11). In aqueous solution, the B
absorption peak showed an obvious blue-shift with the addition
of NaOH. In THF solution, the B absorption peak was almost
Fig. 13 (a) Picture of ordinary paper adsorbed with HPTEO3PA, (b) picture
of ordinary paper adsorbed with HPTEO3PA written with NaOH, (c) picture
of ordinary paper fumigated with hydrochloric acid and (d) picture
of ordinary paper adsorbed with HPTEO3PA rewritten with NaOH. All
pictures were taken under 365 nm UV light.
Fig. 11 UV spectra of HPTEO3PA aqueous solution (blue line),
HPTEO3PA aqueous solution with the addition of NaOH (red line), and
HPTEO3PA THF solution with the addition of solid NaOH. Concentration
(c) = 5 ꢁ 10ꢀ5 mol Lꢀ1
.
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under 365 nm UV light. Thus, the paper could be reused. When
wrote ‘‘XTU’’ on this paper again, the letters ‘‘XTU’’ still emitted
yellow-green fluorescence and showed stronger visual contrast
(Fig. 13d). Thus, the test results indicated that HPTEO3PA can be
used as a rewritable material.
3.5 UV irradiation stimuli-responsive properties of
HPTEO3PA
UV irradiation also affected the properties of HPTEO3PA. Thus,
we initially studied the influence of UV irradiation on the LCST
of HPTEO3PA. Fig. 14(a) shows the temperature–transmittance
curves of the HPTEO3PA aqueous solution treated with 365 nm
UV irradiation for 2 h. HPTEO3PA also exhibited temperature
stimuli-responsive properties even under UV irradiation and its
LCST was enhanced obviously. As observed from Fig. 14(b), the
LCST of all the HPTEO3PA aqueous solutions was enhanced by
1–2 1C after UV irradiation. The enhanced LCST can be
explained as follows: on the one hand, HPTEO3PA changed
from the trans-conformation to cis-conformation with a greater
dipole moment, resulting in an enhancement in its hydrophilicity.
On the other hand, HPTEO3PA can self-assemble into a core–shell
structure with a hydrophilic oligo(ethylene glycol) shell and
hydrophobic a-cyanostilbenic core. After UV irradiation, the
a-cyanostilbenic changed from the trans-conformation into cis-
conformation, resulting in a smaller size of a-cyanostilbenic.
Therefore, the thickness of the hydrophobic core became
smaller. Due to the synergism of these two aspects, the LCST
of the HPTEO3PA aqueous solution was enhanced obviously. A
schematic of this mechanism is shown in Fig. 15.
Fig. 15 Schematic illustration of the conformation transformation of the
HPTEO3PA aqueous solution treated with 365 nm UV irradiation.
when irradiated for 20 min. The emission intensity of this new
peak increased and it shifted to a longer wavelength with an
increase in UV irradiation time until 90 min. The maximum
luminescence wavelength of the new emission peak shifted
from 491 nm to 512 nm and the luminescence intensity was
enhanced 4.83-fold when the UV irradiation time increased
from 20 min to 90 min (Fig. 16c). The blue-shift of the emission
peak located at a shorter wavelength indicated that the trans-
conformation turned into the cis-conformation with an increase
in irradiation time. The enhanced emission intensity of the
peak located at a shorter wavelength may have resulted from the
aggregation of the cis-conformation. The formation of the new
emission peak located at a longer wavelength also evidenced the
aggregation of the cis-conformation and the new emission peak
should be the excimer emission. Some references have been
The photophysical properties of HPTEO3PA showed different
UV irradiation responsive behaviors in water and in THF solution.
In aqueous solution, the luminescence intensity increased first
and remained almost unchanged with an increase in the 365 nm
UV irradiation time (Fig. 16a). With the irradiation of UV light, the
luminescent wavelength of HPTEO3PA shifted to a shorter wave-
length, while its luminescence intensity increased. After irradiation
for 50 min, the luminescent wavelength and the luminescence
intensity was independent of irradiation time. At this moment, the
maximum luminescence wavelength shifted from 442 nm to
417 nm and the luminescence intensity was enhanced 46.4-fold
than before (Fig. 16b). Even more interesting is that a new
emission peak located at a longer wavelength was observed
Fig. 16 (a) Fluorescence spectra of HPTEO3PA aqueous solution with
different UV irradiation times, (b) plots of I/I0 values and the maximum
luminescence wavelength (emission peak located at a shorter wavelength)
versus UV irradiation time for HPTEO3PA aqueous solution and (c) plots of
I/I20 values and the maximum luminescence wavelength (emission peak
located at a longer wavelength) versus 365 nm UV irradiation time for
HPTEO3PA aqueous solution. Excitation wavelength: 365 nm and concen-
Fig. 14 (a) Temperature–transmittance curves of HPTEO3PA aqueous
solution treated with 365 nm UV irradiation for 2 h and (b) LCST of
HPTEO3PA aqueous solution before and after UV irradiation. Measured
wavelength: 600 nm.
tration = 5 mg mLꢀ1
.
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Fig. 17 Pictures of HPTEO3PA aqueous solution with different UV irra-
diation times taken under a 365 nm UV light.
reported that the excimer emission can enhance luminescence
intensity.38,39 However, we first reported UV irradiation-induced
fluorescence enhancement via the production of excimers.
Fig. 17 shows the change in the fluorescence luminescence
behavior of the HPTEO3PA aqueous solution (5 mg mLꢀ1) with
different 365 nm UV irradiation times. The HPTEO3PA aqueous
solution emitted weak blue fluorescence. After irradiation
for 20 min, its fluorescence intensity was enhanced, and the
luminescence wavelength shifted to shorter wavelength. After
irradiation for 2 h, the fluorescence intensity enhanced greatly,
and the wavelength shifted to longer wavelength. Then, the
luminescence color of the HPTEO3PA aqueous solution turned
green. This change is in good agreement with the fluorescence
test results.
Fig. 18 (a) Fluorescence spectra of HPTEO3PA aqueous solution with
different 256 nm UV irradiation times, (b) plots of I/I0 values and the
maximum luminescence wavelength (emission peak located at a shorter
wavelength) versus 256 nm UV irradiation time for HPTEO3PA aqueous
solution and (c) plots of I/I0 values and the maximum luminescence
wavelength (emission peak located at
256 nm UV irradiation time for the HPTEO3PA aqueous solution. Excitation
wavelength: 365 nm and concentration = 5 mg mLꢀ1
a longer wavelength) versus
.
The reversibility of the emission behavior of the HPTEO3PA
aqueous solution was also characterized. The test results are
shown in Fig. 18. As observed from Fig. 18a, the emission peak
located at a longer wavelength had a blue shift and the
emission intensity decreased with irradiation of 256 nm UV
light. After irradiation for 50 min, the maximum luminescence
wavelength shifted from 512 nm to 487 nm and the lumines-
cence intensity decreased to 3% (Fig. 18c). The emission peak
located at a shorter wavelength had a red shift and the emission
intensity decreased with 256 nm UV irradiation time (Fig. 18a).
After irradiation for 110 min, the maximum luminescence
Fig. 19 Fluorescence spectra of HPTEO3PA THF solution (5 mg mLꢀ1
with different UV irradiation times. Excitation wavelength: 365 nm.
)
wavelength shifted from 417 nm to 442 nm and the lumines- The decreased luminescence intensity can also be explained by the
cence intensity decreased to 2.8% (Fig. 18b). This decrease in conformation transformation of HPTEO3PA. The fluorescence
intensity can be explained by the configuration transformation luminescence of HPTEO3PA behaved differently in THF solution
of HPTEO3PA. HPTEO3PA changed from the cis-conformation and in aqueous solution. This is because the cis-conformation
to the trans-conformation when irradiated with 256 nm UV HPTEO3PA molecules resulted from UV irradiation could be
light. It is worth noting that after 256 nm UV irradiation for 110 min, dissolved completely in THF and there was no aggregation for
the emission intensity and the maximum luminescence wavelength the cis-conformation HPTEO3PA molecules. However, HPTEO3PA
of the HPTEO3PAHPTEO3PA aqueous solution reverted to their could self-assemble into core–shell structures in aqueous solution.
original state completely when no 365 nm UV light was applied. When irradiated with UV light, the trans-conformation HPTEO3PA
The test results indicated that the trans-conformation is reversible molecules turned into cis-conformation HPTEO3PA molecules.
and the emission behavior is also reversible. The reversible emission Therefore, the hydrophobic a-cyanostilbenic with the cis-
behavior also excluded the possibility of photochemical reaction and conformation aggregated (Fig. 15). On the one hand, the aggregation
photochemical products.
will induce an enhancement in the fluorescence of the cis-
The fluorescence luminescence behavior of the HPTEO3PA conformation HPTEO3PA molecules. On the other hand, the
THF solution also changed with UV irradiation time. As observed aggregation will lead to the formation of excimers, which will
from Fig. 19, the luminescence intensity of the HPTEO3PA produce a new emission peak and the emission intensity will
THF solution decreased with an increase in UV irradiation time. enhance with an increase in UV irradiation time.
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absorption peak shifted to a shorter wavelength. We also observed
that a new absorption peak appeared at about 250 nm after UV
irradiation. Additionally, the absorption intensity of the new peak
was enhanced with an increase in UV irradiation time. The new
absorption peak should be ascribed to the formation of the cis-
conformation. The test results of the UV absorption spectra of
HPTEO3PA with different UV irradiation times suggested that
365 nm UV irradiation can lead to a conformational trans-
formation in HPTEO3PA in both THF and aqueous solution.
Fig. 20 Size distributions of (a) HPTEO3PA aqueous solution and (b)
HPTEO3PA THF solution with different 365 nm UV light irradiation times.
Concentration = 5 mg mLꢀ1
.
4. Conclusions
To confirm the above explanation, the size distributions of
the HPTEO3PA aqueous solution and HPTEO3PA THF solution
with different UV irradiation times were also studied. As
observed from Fig. 20a, the diameter of the HPTEO3PA aqueous
micelle solution was about 120 nm, which increased greatly
with UV irradiation time.40 When exposed to 365 nm UV light
for 120 min, the diameter increased to about 192 nm. The test
results indicated that HPTEO3PA could self-assemble into
micelles and the diameter of the micelles increased with UV
irradiation time. While in THF solution, the diameter of HPTEO3PA
was only about 2.32 nm. After UV irradiation for 20 min, the
diameter of HPTEO3PA decreased to 1.74 nm (Fig. 20b). These
results suggested that HPTEO3PA cannot form aggregates in
THF solution and the molecular size of HPTEO3PA decreased
when HPTEO3PA changed from the trans-conformation to the cis-
conformation under the effect of UV irradiation. The difference in
the diameter change between the HPTEO3PA aqueous solution
and HPTEO3PA THF solution treated with different UV irradiation
times indicated that the enhanced emission intensity by UV
irradiation originates from the formation of aggregates.
The UV absorption spectra of HPTEO3PA with different UV
irradiation times in aqueous solution and in THF solution were
characterized. As observed from Fig. 21, the maximum UV absorp-
tion peak of the HPTEO3PA THF solution and HPTEO3PA aqueous
solution shifted to a shorter wavelength and its absorption intensity
decreased after treatment with 365 nm UV irradiation. The blue-shift
of the maximum UV absorption peak should be attributed to
conformation transformation of HPTEO3PA. When irradiated with
UV light, the trans-conformation changed to the cis-conformation,
resulting in a more twisted conformation. Therefore, the conjugation
degree of the whole molecule decreased and the maximum
Herein, we prepared a novel a-cyanostilbenic-based fluoro-
phore. The synthetic compound HPTEO3PA exhibited obvious
AIEE properties, which emitted weak fluorescence in a good
solvent while its emission intensity was enhanced greatly in the
aggregate state. HPTEO3PA also showed stimulus-responsive
behavior to temperature, pH and UV irradiation. Firstly, it
showed thermosensitivity. HPTEO3PA was soluble in water at
a lower temperature, while insoluble at a higher temperature. The
luminescence behavior of HPTEO3PA also showed temperature
sensitivity and its luminescence intensity showed the trend
of declining–rising–declining with an increase in temperature.
Secondly, HPTEO3PA showed pH sensitivity. In aqueous solution,
the LCST of HPTEO3PA decreased first and then increased with
the addition of alkali. The luminescence intensity increased
slightly with the addition of alkali. While in THF solution, the
luminescence intensity was enhanced greatly with the addition of
alkali and the maximum luminescence wavelength exhibited
different degrees of red-shifting with the addition of different
alkalis, which could be used as probe for alkali. HPTEO3PA could
also be used as a rewritable material. Lastly, HPTEO3PA showed
UV irradiation sensitivity. In THF solution, its luminescence
intensity decreased with an increase in UV irradiation time. In
aqueous solution, its luminescence intensity increased with an
increase in UV irradiation time. Its luminescence color changed
from blue to green after irradiation for 2 h. This study offers
a new method to prepare AIEE-active multistimuli-responsive
fluorescent materials.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by National Natural Science Foundation of
China (No. 51573152, 51373148). Open Fund Project of Innovation
Platform in Colleges and Universities of Hunan Province (16K085).
Notes and references
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Fig. 21 UV spectra of HPTEO3PA THF solution (a) and HPTEO3PA
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5 ꢁ 10ꢀ5 mol Lꢀ1
.
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