Synthesis, characterization and visible light activated fluorescence of azo caged aggregation-induced emission polymers

Article abstract of DOI:10.1016/j.dyepig.2019.107569

Tetraphenylethylene (TPE) based aggregation-induced emission (AIE) polymers containing different degrees of functionalization of photobleachable azo chromophores were developed and their photophysical properties were explored. The synthesized epoxy monome

Full text of DOI:10.1016/j.dyepig.2019.107569

Accepted Manuscript
Synthesis, characterization and visible light activated fluorescence of azo caged
aggregation-induced emission polymers
Bing Wu, Jiajia Shen, Wei Wang, Tianhao Xue, Yaning He
PII:
S0143-7208(19)30726-0
DOI:
https://doi.org/10.1016/j.dyepig.2019.107569
Article Number: 107569
Reference:
DYPI 107569
To appear in: Dyes and Pigments
Received Date: 3 April 2019
Revised Date: 13 May 2019
Accepted Date: 19 May 2019
Please cite this article as: Wu B, Shen J, Wang W, Xue T, He Y, Synthesis, characterization and visible
light activated fluorescence of azo caged aggregation-induced emission polymers, Dyes and Pigments
(2019), doi: https://doi.org/10.1016/j.dyepig.2019.107569.
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Graphical Abstract
Synthesis, Characterization and Visible Light Activated Fluorescence of
Azo Caged Aggregation-induced Emission Polymers
Bing Wu, Jiajia Shen, Wei Wang, Tianhao Xue and Yaning He*
Visible light activated fluorescence of imidazole-type azo caged aggregation-induced
emission polymers was reported. The effect of azo contents on fluorescence intensity was
studied, which revealed that only a small amount of imidazole-type azo groups could totally
cage the strong AIE fluorescence of the polymers. Irradiated with visible light, azo
chromophores could be photo bleached, leading to remarkable recovery of the blue
fluorescence. Fluorescent patterns have been easily fabricated.
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Synthesis, Characterization and Visible Light Activated
Fluorescence of Azo Caged Aggregation-induced Emission
Polymers
Bing Wu, Jiajia Shen, Wei Wang, Tianhao Xue and Yaning He*
Department of Chemical Engineering, Key Laboratory of Advanced Materials (MOE), Tsinghua
University, Beijing 100084, China
*Corresponding author: Tel. +86 01 62784561
E-mail: heyaning@mail.tsinghua.edu.cn
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Tetraphenylethylene (TPE) based aggregation-induced emission (AIE) polymers
containing different degrees of functionalization of photobleachable azo
chromophores were developed and their photophysical properties were explored. The
synthesized epoxy monomer derived from TPE core reacted with aniline to give the
precursor polymer, which confirmed typical AIE characteristics. Then through a
post-polymerization azo-coupling scheme, various contents of 2-phenylazo-4,
5-dicyanoimidazole groups were readily introduced into the TPE based epoxy
precusor polymer. Experimental results suggested that only a small amount of such
imidazole-type azo component could totally cage the intense AIE fluorescence of the
whole polymer due to the fluorescence resonance energy transfer process. By
exposure to visible light (450 nm), azo units could be photo bleached notably, thus
resulting in dramatic recovery of the blue fluorescence. Depending on this, visible
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light induced fluorescent patterns were easily fabricated and erased, indicating that the
prepared azo caged AIE Polymers could be promising candidates for
anti-counterfeiting and optical information storage.
26 Keywords: Aggregation-induced emission; Azo caged; Tetraphenylethylene;
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Visible light; Fluorescent polymer
29 1 Introduction
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In recent years, stimuli-responsive caged fluorescent systems, whose emission is
quenched by a quencher and recovers upon cleavage of the quencher under certain
stimuli, have attracted considerable attention for their promising prospect in sensors,
optical recording devices, etc [1-4]. Anyway, traditional fluorescent compounds
generally suffer from the aggregation-caused quenching (ACQ) problem in
aggregated or solid state. In 2001, Tang’s group proposed aggregation-induced
emission (AIE) first to address this thorny issue, leading to new insights into the
development of organic luminophores [5-16]. Due to AIEgens’ good photostability
and high photobleaching resistance in aggregated state, caged AIE materials with
stimuli-responsive strategies have been extensively reported, which can be employed
in numerous applications ranging from chemical sensing to bioprobes and so on
[17-20]. Among diverse stimulus techniques, light is recognized to enjoy plenty of
unique advantages. For example, it is exceptionally convenient for light to adjust
irradiating position, intensity, beginning and ending time as well as specific
wavelengths. However, photoactivatable AIE systems are still rare. Tang et al. have
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investigated a new caged fluorophore obtained by connecting a tetraphenylethylene
(TPE) derivative and a 2-nitrobenzyl group [21]. The caged compound can be
photoactivated and induced to emit strong cyan fluorescence by ultraviolet (UV)
exposure. Anyhow, UV light has very high energy, which can be damaging to
organisms and result in degradation for scores of macromolecules. To utilize lower
energy light resource such as visible light will be a much better choice [22]. In
particular, for the caged AIE compound, one fluorescent unit will be often linked with
one quencher, thus the fluorescence resonance energy transfer process can take place
well. However, one quencher may quench more than one fluorescent group. In terms
of this point, the caged AIE polymer may manifest better sensitivity to the stimuli.
Furthermore, AIE polymers hold much more advantages in practical applications for
their simple fabrication of large area films. Therefore, facile construction of
photoactivatable AIE polymers responding to visible light is reasonably important but
scarce.
Azo chromophores have been well known for their photoinduced reversible
isomerization upon irradiation with light at appropriate wavelength, which promises
potential in varying areas such as biological imaging, photoswitching, and many
others [23-31]. Besides photochromic behaviours, azo chromophores also have been
used as non-fluorescent energy acceptors, which can efficiently quench the
fluorescence of the system. We have found that azo groups could be effectively
bleached by enzyme or chemical reductant [32-34], causing great changes of both
ultraviolet-visible (UV-vis) absorption spectra and fluorescence emission spectra. In
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addition to the above biological or chemical method, some kind of azo chromophore
has shown photobleachable performance with visible light actually [35]. On the basis
of this, a class of visible light triggered AIE-active polymers with various
photobleachable azo chromophore contents was synthesized and fully characterized in
this work. TPE was implemented as the fluorescent building block in the backbone of
epoxy based polymers owing to brief synthesis procedures and easy modifications.
Imidazole-type azo chromophores were chosen as photoquenching and
photobleaching sites. The photophysical properties of azo caged AIE polymers were
sufficiently discussed. Upon irradiation with the light at the wavelength of 450 nm,
the absorption band appearing in the visible range (λmax at 466 nm) ascribed to the π–
π∗ electron transition of azo chromophores decreased noticeably, which was caused by
photobleaching without any addition of external reducing agents. Simultaneously, the
fluorescent emission of the polymer increased sharply. Furthermore, the fluorescent
pattern on the spin-coated film of the prepared azo caged AIE polymer was fabricated
and erased by exposure to visible light conveniently, which might be exploited in
anti-counterfeiting and optical information storage.
83 2 Experimental Section
84 2.1 Materials and characterization
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The purification of tetrahydrofuran (THF) was accomplished by distillation with
metal sodium and benzophenone under argon atmosphere prior to use. Ultrapure
water with resistivity > 18 MΩ·cm was used in the experiments, which was obtained
from a Milli-Q water purification system. All other solvents and starting materials
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were of analytical grade and purchased from commercial sources, which were directly
used without additional purification.
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1H NMR and C NMR spectra were conducted with a JEOL JNM-ECA 600 NMR
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spectrometer by using deuterated chloroform (CDCl3) or deuterated dimethyl
sulfoxide (DMSO-d6) as the solvent and tetramethylsilane as the internal standard.
UV-vis absorption spectra were determined by using an Agilent 8453
spectrophotometer. Fourier transform infrared (FTIR) measurements were conducted
using an IRTracer-100 spectrometer (Shimadzu). Fluorescence emission spectra were
measured on a Hitachi F-7000 fluorescence spectrophotometer. The weight-average
molecular weight (Mw), the number-average molecular weight (Mn) and relative
molecular mass distribution (MWD) were determined on a gel permeation
chromatography (GPC) apparatus with THF as eluent at a flow rate of 1.0 mL·min-1.
Equipped with a refractive index detector and fitted with a PL gel 5 mm mixed-D
column, the instrument was calibrated by using linear polystyrene (PS) standards.
Photoirradiation (450 nm) was performed on a led curing system (IWATA UV-101D)
and the formed pattern was observed with a 365 nm UV lamp. The intensity of 450
nm visible light is about 150 and 500 mW·cm2 for the photobleaching of TPE-IZ-DC
with a DF of 2.5% and 38%, respectively. Mass spectra data were recorded on a
LCMS-IT/TOF (Shimadzu). Laser light scattering (LLS) measurements were
performed on an ALV/DLS/SLS-5022F spectrometer with a multi-τ digital time
correlator (ALV/LSE-5003) and a 17 mW solid-state laser (λ = 632 nm) as the light
source.
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111 2.2 Synthesis of TPE-2OH
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To a solution of 4-hydroxybenzophenone (3.97 g, 0.02 mol) in dry THF (80 mL) was
added Zn dust (7.85 g, 0.12 mol). The mixture was stirred vigorously for a quarter of
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an hour to cool to 0 C. Into the above mixture was dropwise added titanium
tetrachloride (6.6 mL, 0.06 mol) and stirred in an ice water bath for 1 hour. Then it
was gradually heated to reflux under argon atmosphere for 24 hours. The reaction was
allowed to be cooled to room temperature and quenched with 10% K2CO3 aqueous
solution and the newly formed precipitate was removed. The residue was extracted
three times with 100 ml CH2Cl2, dried, filtered. After all of the solvent CH2Cl2 has
been evaporated under reduced pressure, the target product was further purified by
silica-gel column chromatography (300-400 mesh, eluent ethyl acetate / petroleum
ether 1:2 vol). White solid in 53% yield. 1H NMR (600 MHz, CDCl3), δ (TMS, ppm):
7.13-6.98 (m, 10H), 6.91-6.85 (m, 4H), 6.59-6.54 (m, 4H). 13C NMR (150 MHz,
CDCl3), δ (TMS, ppm): 154.03, 144.28, 144.17, 139.82, 139.80, 136.79, 136.71,
132.87, 132.85, 131.51, 131.50, 127.82, 127.72, 126.38, 114.79, 114.71. MS(ESI) m/z:
[M-H]- calcd C26H20O2 363.1391, found 363.1387.
127 2.3 Synthesis of TPE-2EP
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A mixture of TPE-2OH (1 g, 2.74 mmol), epichlorohydrin (5.07 g, 54.8 mmol) and
isopropyl alcohol (35 ml) was sequentially added into 100 mL three-necked
round-bottom flask equipped with a magnetic stirrer and refluxed. Then 3.5 ml 20%
sodium hydroxide aqueous solution was dropwise added into the mixture over half an
hour. The reaction mixture stirred at reflux temperature under argon atmosphere for 5
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hours. After cooling to room temperature, the mixture was poured into chloroform and
thoroughly washed with ultrapure water till neutrality. The combined organic layer
was dried over magnesium sulfate, filtered, and chloroform was removed by rotatory
evaporation under reduced pressure. The obtained crude product was further purified
by column chromatography over silica gel (300-400 mesh) using ethyl acetate /
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petroleum ether (1:3 vol) as eluent. Yellow solid in 34% yield. H NMR (600 MHz,
CDCl3), δ (TMS, ppm): 7.12-7.07 (m, 6H), 7.03-7.01 (m, 4H), 6.92-6.90 (m, 4H),
6.64-6.63 (m, 4H), 4.14-4.11 (m, 2H), 3.89-3.87 (m, 2H), 3.32-3.30 (m, 2H),
2.89-2.87 (t, J = 4.2 Hz, 2H), 2.73-2.71 (m, 2H). 13C NMR (150 MHz, CDCl3), δ
(TMS, ppm): 156.98, 144.22, 139.83, 137.00, 132.69, 131.48, 127.83, 126.39, 113.81,
68.67, 50.21, 44.91. MS(ESI) m/z: [M] calcd C32H28O4 476.1982, found 476.1928.
144 2.4 Synthesis of TPE-AN
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A mixture of TPE-2EP (1 g, 2.10 mmol) and phenylamine (0.1955 g, 2.10 mmol) was
added into 10 mL Schlenk flask equipped with a magnetic stirrer. After degassing and
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recharging with argon, the solid was heated to be fused at 150 C and kept one hour.
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Subsequently, the reaction mixture was heated at 110 C by stirring for another 24
hours. After the completion of the reaction, the resulting crude product was dissolved
into a minimal amount of dimethyformamide (DMF) and then precipitated with
excess petroleum ether to remove the unreacted TPE-2EP and phenylamine. The
product was isolated by filtration, vacuum dried at 65 oC over 24 hours for the use in
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further post-polymerization azo-coupling scheme. White powder in 78% yield. H
NMR (600 MHz, DMSO-d6), δ (TMS, ppm): 7.12-7.08 (m, 8H), 6.97-6.93 (m, 4H),
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6.86-6.82 (m, 4H), 6.71-6.67 (m, 6H), 6.59-6.51 (m, 1H), 5.33 (m, 1H), 5.15 (m, 1H),
4.01 (m, 2H), 3.83 (m, 4H), 3.68-3.66 (m, 1H), 3.55-3.53 (m, 1H), 3.43-3.40 (m, 1H),
3.30-3.27 (m, 1H). GPC: Mn = 13000 g·mol-1; Mw = 25700 g·mol-1; MWD = 1.98.
158 2.5 Synthesis of typical azo polymers TPE-IZ-DC
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A series of azo polymers TPE-IZ-DC with different degrees of functionalization of
azo chromophores were synthesized through a post-polymerization azo-coupling
scheme. The brief procedures are as follows. To
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solution of
2-amino-4,5-imidazoledicarbonitrile (0.319 g, 2.4 mmol) and 9 mL glacial acetic acid
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was dropwise added 0.9 mL concentrated sulfuric acid at 0 C. Then into the above
white mixture was dropwise added sodium nitrite (0.215 g, 3.12 mmol) in 0.9 mL of
water to give diazonium salts of 2-amino-4,5-imidazoledicarbonitrile. A mixture of
TPE-AN (100 mg) and 5 mL DMF was added into 25 mL round-bottom flask
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equipped with a magnetic stirrer and cooled to 0 C. And then, a certain amount of
diazonium salts was added dropwise into the above TPE-AN solution while the
temperature was maintained at 0 oC. The colourless solution turned to red immediately.
After being vigorously stirred overnight, the mixture was poured into excess saturated
brine. The pure product TPE-IZ-DC with specific azo content was filtered, vacuum
dried at 70 oC for 24 hours.
173 2.6 Preparation of the TPE-IZ-DC film
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The substrates (quartz glass slides) were ultrasonically cleaned in sodium hydroxide
concentrated solution, ultrapure water and acetone, successively. And then they were
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dried at 70 C prior to use. A homogeneous solution of TPE-IZ-DC (50 mg) and
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DMF (0.5 mL) was filtered with 0.25 µm filter. After that, it was spin-coated onto
substrates with a spin speed of 1500 rpm, followed by vacuum drying at 60 oC for 24
hours.
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180 2.7 Visible light response of the TPE-IZ-DC film and fluorescent pattern
181 fabrication
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TPE-IZ-DC films were vertically exposed to visible light (450 nm) at room
temperature to investigate the visible light response every thirty minutes with UV-vis
absorption spectrophotometer and fluorescence emission photometer. Fluorescent
patterns on the spin-coating films were fabricated by irradiating with the same light
source for 150 minutes through a photomask. Fluorescence images of the above
patterns were recorded under a 365 nm UV lamp.
188 3 Results and Discussion
189 3.1 Synthesis of azo caged AIE polymers TPE-IZ-DC
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A set of visible light triggered AIE polymers TPE-IZ-DC containing different mole
fractions of azo groups was achieved and the synthetic route of target polymers
TPE-IZ-DC was illustrated in Scheme 1. According to the molecular design,
synthesis of azo caged polymers TPE-IZ-DC was facile with a four-step synthetic
approach. 1,2-bis(4-hydroxyphenyl)-1,2-diphenylethene (TPE-2OH), an iconic
AIEgen, was in a one-step preparation starting from 4-hydroxybenzophenone through
a McMurry coupling reaction. The TPE based epoxy compound (TPE-2EP) was
prepared by incorporating epoxy groups into TPE-2OH. Then synthesis of the
precursor polymer TPE-AN was done by the polycondensation of difunctional
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epoxide and phenylamine monomers at relatively low temperature. The molecular
weight and molecular weight distribution of the linear polymer TPE-AN were
determined by GPC method and satisfying results were presented in Fig.S7. The
number-average molecular weight and relative molecular mass distribution of the
precursor polymer TPE-AN were 13000 g·mol-1 and 1.98, respectively. Finally, the
target polymers TPE-IZ-DC were synthesized by the post-polymerization
azo-coupling reaction between polymer TPE-AN and diazonium salts. The reaction
proceeded efficiently as expected and the final products TPE-IZ-DC displayed
glorious solubility in common organic solvents. Further details could be found in the
Experimental Section and Supporting Information.
1H NMR and 13C NMR spectra suggested successful synthesis of all compounds,
which were included in the Supporting Information (Fig.S1 - 6). As could be seen
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from H NMR spectra of TPE-AN and TPE-IZ-DC (Fig.S5 and Fig.S6), the
incorporation of 2-phenylazo-4, 5-dicyanoimidazole parts has been identified by
combining information as follows. First, peak areas of protons at para-position of
anilino groups whose chemical shifts are around 6.55 ppm relatively reduced in the
whole low magnetic field corresponding to all aromatic rings. Second, the signal
newly appeared at 7.75 ppm was attributed to benzenoid protons at meta positions of
amino segments, which was caused by formation of electron-withdrawing azo groups.
We defined that degrees of functionalization (DFs) of azo chromophores were molar
ratios between 2-phenylazo-4, 5-dicyanoimidazole groups in the final polymer chain
and anilino pendants of the precursor polymer TPE-AN. Benefiting from efficient azo
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coupling reaction, DFs could be controlled by selecting suitable amount of diazonium
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salts. The DFs have been estimated from H NMR by comparing peak areas of
protons of azo units and the others in the aromatic region.
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Scheme 1. Synthetic route of azo caged AIE polymers TPE-IZ-DC.
226 3.2 AIE effect of the precursor polymer TPE-AN
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To explore AIE activity of the precursor polymer TPE-AN, fluorescence emission
spectra were recorded in different ratios of water-DMF mixtures (Fig.1a). DMF was
selected as the good solvent because TPE-AN could be resolved adequately and
soluble with the poor solvent water at any proportion. When a little water was added
to the DMF solution of TPE-AN (water proportion is lower than 10%), it was not
emissive. The two spectra (0% and 10%) seemed to overlap each other. With
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increasing the water percentage to 40%, blue fluorescence sharply enhanced, which
resulted from vast aggregation formation. After that, the maximum intensity remained
the same. The depict changes in fluorescence relative peak intensity (I/I0) were shown
in Fig.1b. The relative intensity ratio (water contents are 90% and 0%, respectively)
was as high as 155, thus testifying its AIE nature. In the light of the restriction of
intramolecular motion (RIM) mechanism, aggregates of hydrophobic fluorescent
materials were increased with increasing poor solvent water. The aggregation of the
polymer was confirmed by Tyndall phenomenon and LLS. As water content was 90%,
an obvious Tyndall phenomenon emerged (Fig.S8), comparing with that in pure DMF.
LLS results also confirmed the formation of TPE-AN aggregates (Fig.S9). As water
content reach 98%, suspended solids were seen visually. As a result, TPE-AN could
be highly emissive in the aggregated state with maximum emission wavelength of 472
nm. Similar results were also found in the solid film of TPE-AN (Fig.S10). The
intense emission of the film also exhibited typical AIE feature. In addition, the
blueshift of emission peak position from aggregates to the solid film was about 5 nm.
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Fig. 1. (a) Changes in fluorescence emission spectra of dilute solutions of TPE-AN in water-DMF mixtures with different water fractions.
(b) Correlation between the relative emission peak intensity (I/I0) at 472 nm of TPE-AN in water-DMF mixtures and water percentage.
Concentration of TPE-AN: 0.01 mg/mL; excitation wavelength: 340 nm.
253 3.3 The relationship between absorption intensity of TPE-IZ-DC and
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254 DFs
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Besides fluorescence spectrometry, UV-vis absorption spectra of the polymer
TPE-AN was also recorded to study its optical property, corresponding to
TPE-IZ-DC with 0% DF, the black line in Fig.2a. No absorption peaks appeared in
the visible region for TPE-AN. For imidazole-containing polymers TPE-IZ-DC with
other DFs, they exhibited significant absorption ranging from 370 nm to 600 nm in
line with expectations, which also confirmed the introduction of heteroaromatic azo
moieties into polymer chains of TPE-AN successfully. The absorption maxima
appearing at 466 nm resulted from π-π∗ electron transition of pseudo-stilbene type azo
chromophores. Fig.2b showed that the absorption maxima of TPE-IZ-DC in
water-DMF mixtures with 90% water fraction increased with the DFs.
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Fig. 2. (a) Changes in UV-vis absorption spectra of dilute solutions of TPE-IZ-DC with different DFs in water-DMF mixtures (90% water
fraction). (b) Correlation between the relative absorption peak intensity (I/I0) at 466 nm of TPE-IZ-DC and DFs in water-DMF mixtures
(90% water fraction). Concentration of TPE-IZ-DC: 0.01 mg/mL.
270 3.4 Fluorescence quenching effect of TPE-IZ-DC
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Since the absorption band of azo units almost covered all of emission region of TPE
components proved in Fig.S11, fluorescence resonance energy transfer process would
work and fluorescence of the polymers TPE-IZ-DC would be faded (Fig.3a). To
verify the quenching efficiency, the influence of azo contents on fluorescence
intensity was investigated. Fig.3b gives the curve of relative emission peak intensity
at 472 nm of azo-containing polymers TPE-IZ-DC in water-DMF mixtures with 90%
water fraction vs. DFs. It turned out as expected that the maximum value of emission
intensity apparently decreased almost one order of magnitude by only 2.5% azo
contents, implying excellent quenching efficiency of fluorescence.
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Fig. 3. (a) Changes in fluorescence emission spectra of dilute solutions of TPE-IZ-DC with different DFs in water-DMF mixtures (90%
water fraction). (b) Correlation between the relative emission peak intensity (I/I0) at 472 nm of TPE-IZ-DC in water-DMF mixtures (90%
water fraction) and DF. Concentration of TPE-IZ-DC: 0.01 mg/mL; excitation wavelength: 340 nm.
285 3.5 Visible light response of the TPE-IZ-DC film
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transparence and favourable film-forming property. On the grounds of these,
TPE-IZ-DC films were obtained using the spin-coating method and visible light
response over different durations of light irradiation was measured by UV-vis
absorption spectra at first, as described in Fig.4. Given the convenience of
photobleaching, azo amount in the polymer was set at 2.5%. It has been demonstrated
that absorption maxima at 469 nm of the TPE-IZ-DC film reduced prominently under
continuous irradiation at the wavelength of 450 nm. Within only 30 minutes,
absorption intensity of residual azo functional groups was rapidly lost approximately
50%. After illumination for 150 minutes, it was only ninth of the originally state,
indicating a sensitive response to visible light as well as a high signal-to-noise ratio.
For pseudo-stilbene type azo chromophores, cis state is unstable and will rapidly relax
to the trans form. As shown in Fig.5, the absorption intensity at 469 nm from
TPE-IZ-DC decreased with the increase of irradiation time and the absorption peak
of TPE-IZ-DC upon irradiation was not recoverable at all even though we left the
film in the dark for a long time or heated. Besides, the irreversible color change of the
film was easily seen from red to almost colorless with the naked eye. It should be
considered that azo chromophores ultimately disappeared by photobleaching instead
of photoisomerization. FTIR spectra were also used to prove this photobleaching
process. As presented in Fig.S12, several characteristic peaks in FTIR spectra
assigned to absorption bands of TPE-IZ-DC (DF = 38%) decreased apparently, which
could be also postulated the photobleaching of azo chromophores after exposure to
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Fig. 4. (a) Time dependent absorption changes of the TPE-IZ-DC film irradiating with visible light at the wavelength of 450 nm. (b)
Correlation between the relative absorption peak intensity (I/I0) at 469 nm of the TPE-IZ-DC film and exposure time.
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As expected, changes of absorption intensity of the TPE-IZ-DC film greatly affected
its fluorescence performance. Fig.5 clarified the blue fluorescence recovery after
photobleaching by the same process of irradiation with visible light monitored by
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fluorescence emission spectra. It could be found that characteristic emission definitely
increased as exposure time increased, in accordance with the UV-vis absorption
spectra. After irradiating the film with the same light source for 150 minutes,
photobleaching caused an increase in maximum emission intensity at 467 nm by more
than six times since fluorescence resonance energy transfer process did not work
anymore.
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Fig. 5. (a) Time dependent fluorescence intensity changes of the TPE-IZ-DC film irradiating with visible light at the wavelength of 450
nm. (b) Correlation between emission peak intensity (I/I0) at 467 nm of the TPE-IZ-DC film and exposure time.
326 3.6 The fluorescent pattern fabrication
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The above results that visible light possessed the ability to efficiently blench azo
chromophores and fluorescent emission was enhanced concomitantly impelled us to
make an attempt at fabricating fluorescent patterns by the mask method. These
observed patterns on the basis of photobleaching were transcribed from designed
photomasks. The feasibility and simplicity of the valid and powerful technique were
proved with an example. The mask was positioned onto the TPE-IZ-DC film, and
then the film was exposed to visible light using 450 nm light source for 150 minutes.
It was satisfying that the fluorescence of illumination field was brighter than its
surroundings. The letter pattern “FL” as a representative emerged as displayed in
Fig.6. After further irradiation, the pattern was almost completely erased. Due to this
successful demonstration, large-area or/and complicated patterns would be easily
tailored when films of the desired size and corresponding masks were applied, making
TPE-IZ-DC potentially suitable for optical information storage materials, antifake
materials and other application areas.
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Fig. 6. The photographs of a piece of untreated film (a), the fluorescent pattern fabricated on exposed areas by visible light irradiation
process (b) and the pattern erasure after removing the mask under continuous irradiation (c) taken under a 365 nm UV lamp.
344 4 Conclusions
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Azo caged aggregation-induced emission polymers were synthesized and their
fluorescence performance activated by visible light was fully characterized. The effect
of azo contents on fluorescence intensity was studied, which revealed that only a
small amount of imidazole-type azo groups could totally cage the strong AIE
fluorescence of the polymers due to the fluorescence resonance energy transfer
process. Irradiated with visible light, azo chromophores could be photo bleached,
leading to remarkable recovery of the blue fluorescence. Visible light induced
fluorescent patterns have been fabricated and erased, which indicated that the
prepared azo caged AIE Polymers could especially competitive in the material design
of anti-counterfeiting and optical information storage.
355 Declaration of Interest
356
The authors declare no competing financial interest.
357 Acknowledgements
358
359
360
The authors gratefully acknowledge financial support from the National Key R&D
Program of China (Grant No.2017YFA0701303), and the National Natural Science
Foundation of China (Grant Nos. 51873097 and 21674058).
361 References
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Highlights
1. Imidazole-type azo caged aggregation-induced emission polymers were
synthesized.
2. Only a small amount of imidazole-type azo groups could totally cage the strong
AIE fluorescence of the polymers.
3. Imidazole-type azo chromophores of the polymers could be photo bleached with
visible light, leading to activating their fluorescence performance.
4. Visible light induced fluorescent patterns have been easily fabricated and erased.