Photoresponsive aggregation-induced emission polymer film for anti-counterfeiting

Article abstract of DOI:10.1016/j.cclet.2021.05.053

The development of solid-state smart materials, in particular those showing photoresponsive luminescence, is highly desirable for their cutting edge applications in displays, sensors, data-storage, and anti-counterfeiting. However, to achieve both excelle

Full text of DOI:10.1016/j.cclet.2021.05.053

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Photoresponsive aggregation-induced emission polymer film for
anti-counterfeiting
Hong Yang , Mengqi Li , Weijun Zhao , Zhiqian Guo ,
Wei-Hong Zhu
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https://doi.org/10.1016/j.cclet.2021.05.053
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Chinese Chemical Letters
Please cite this article as: Hong Yang , Mengqi Li , Weijun Zhao , Zhiqian Guo , Wei-Hong Zhu ,
Photoresponsive aggregation-induced emission polymer film for anti-counterfeiting, Chinese Chemical
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Communication
Photoresponsive aggregation-induced emission polymer film for anti-counterfeiting
Hong Yang, Mengqi Li, Weijun Zhao, Zhiqian Guo, Wei-Hong Zhu
Shanghai Key Laboratory of Functional Materials Chemistry, Key Laboratory for Advanced Materials and Institute
of Fine Chemicals, Joint International Research Laboratory of Precision Chemistry and Molecular Engineering,
Feringa Nobel Prize Scientist Joint Research Center, School of Chemistry and Molecular Engineering, East China
University of Science & Technology, Shanghai 200237, China
 Corresponding author.
E-mail address: zhwj@ecust.edu.cn (W. Zhao).
———
 Corresponding author.
E-mail address: zhwj@ecust.edu.cn (W. Zhao).

Graphical Abstract
Integrating steric-hindrance diarylethene into flexible polymer chains enables reversibly solid-state photochromism
and excellent AIE activity. Such fascinating polymer film with phototransformation between invisible and visible is
successfully established in anti-counterfeiting.

ABSTRACT
The development of solid-state smart materials, in particular those showing photoresponsive luminescence, is
highly desirable for their cutting edge applications in displays, sensors, data-storage, and anti-counterfeiting.
However, to achieve both excellent photoresponsive performance and bright luminescence in solid state remains
challenge. Herein, we integrate a novel photochromic fluorophore YL into flexible polymer chains, thereby enabling
the resultant polymer PYL with reversible photoisomerization upon aggregation. Remarkably, the polymer PYL
possesses excellent photochromic properties and aggregation-induced emission (AIE) activity, which can be
attributed to the photoactive YL moiety. Upon light exposure, its film exhibits reversibly off-to-on fluorescent
modulation with quick response, high emission efficiency and signal contrast, sharply different from the weak
emission in solution. The novel photoresponsive AIE polymer with invisible/visible color and fluorescence
transformation allows for advanced anti-counterfeiting applications. This work provides an efficient platform for
constructing solid-state photocontrollable luminescent materials.
Keywords:
Aggregation-induced emission; Photoresponsive luminescence; Diarylethene; Photochromism; Polymer film; Anti-
counterfeiting

Solid-state smart materials with stimulus responsive luminescence have attracted increasing attention for a variety of applications,
such as displays [1-3], environmental monitoring [4-6], bio-imaging [7-9], and anti-counterfeiting [10-12]. Of particular interest are
those photoresponsive luminescent materials, as light is more attractive to realize high-precise spatiotemporal control in a non-invasive
way [13-15]. As the most widely utilized photoresponsive units, diarylethenes are capable of undergoing reversible photo-triggered
structural isomerization with fast response, excellent thermal bistability and fatigue resistance [16-19]. Although encoding
fluorophores into diarylethenes has been widely utilized to achieve photo-induced fluorescence regulation [20-22], this strategy always
faced with complicated synthetic route and unavoidable trade-off between fluorescence and photochromic properties [23]. And these
encoded diarylethenes have limited free space for solid-state photoreactions or faced with aggregation-induced quenched emission
which greatly hampers their fluorescent switching in solid state and limits practical applications [24-28].
Recently, our group has developed series of steric-hindrance photochromic systems which possess reversible off-to-on fluorescent
photoswitching performance in film with high efficiency and excellent fatigue resistance [29]. But its powder failed to undergo
efficient photocyclization reaction mainly since that the rigid environment upon aggregation greatly limited the intermolecular space
[30]. The doped method indeed provided more free volume to afford the photocyclization process, but would easily bring uneven
dispersion without isotropy and precipitation by crystallization. Instead, covalently linking photochromophore to flexible polymer
chains can not only address the above problems, but also make the resultant polymer possesses good processability [31-34], which is
conducive to the further practical application.
Accordingly, we here incorporated steric-hindrance photochromic unit YL into flexible polymer chains through radical
copolymerization (Fig. 1). The resultant polymer PYL displayed both excellent aggregation-induced emission (AIE) and reversibly
photoresponsive properties in solid state with high photocyclization quantum yield, thermal bistability and fatigue resistance. Upon
UV/visible light irradiation, PYL film exhibits reversibly off-to-on fluorescent and obvious color switching with quick response and
high contrast. Such polymer film and its fascinating photoresponsive color and luminescent properties allow high-tech applications in
advanced anti-counterfeiting.
As the reported photochromic core compound BBTE has photoactive antiparallel (ap-) and photoinactive parallel (p-) conformers
which were successfully separated [35], we synthesized pure ap-YL and p-YL by the Suzuki coupling reaction (Scheme S1 in
Supporting information). Indeed, the conversion between ap-YL and p-YL was totally blocked (Scheme S2 in Supporting information)
under room temperature, according to the NMR spectra. All chemical structures were characterized by 1H and 13C NMR spectroscopy
as well as high-resolution mass spectra (HRMS). Then, the target polymer PYL (Scheme S1) was obtained by the radical
polymerization of photoactive ap-YL and NIPAM unit. With polystyrene as a standard and THF as solvent, the number-average
molecular weight Mn and polydispersity Mw/Mn of PYL is 40428 and 1.58 (Fig. S18 in Supporting information), respectively, measured
by GPC, indicative of a narrow molecular weight distribution. The molar ratio of photochromic moiety and NIPAM units was about 1:
907, calculated from the absorption spectra (Fig. S19 in Supporting information) [36,37].
Fig. 1. Design strategy of photoresponsive AIE polymer. Light-modulated luminescence switching of photochromophores in different states, including the
photoactive monomer YL (a) in solution, (b) in aggregate state, and (c) polymer PYL in aggregate sate. The structure of photoresponsive unit YL and polymer
unit are shown on the right.
In case of photochromic diarylethenes, only ap-conformer can undergo photocyclization reaction from open to closed isomer [38,39],
thus we studied the photoswitching behavior of ap-YL and p-YL individually in solution. As expected, pure p-YL showed neither
absorption nor emission change upon light irradiation (Fig. S20 in Supporting information). In contrast, the initial colorless THF
solution of ap-YL turned red accompanied by a new absorption band centered at 552 nm (Fig. 2a), which is identical to its closed

isomer (c-). The photocyclization conversion ratio was up to 98% (Table S1 in Supporting information). Meanwhile, we noticed that
ap-YL in THF solution displayed weak fluorescence centered at 544 nm (Table S2 in Supporting information), resulting from the
intramolecular charge transfer process due to its donor-π-acceptor structure. This emission gradually quenched until reaching
photostationary state (pss-). And then under visible light ( > 510 nm), it totally returned to the initial state with recovered emission.
During this reversible process, the photocyclization and photocycloreversion quantum yield was 0.70 and 0.05, respectively. In
addition, YL showed excellent thermal bistability at 293 K and 318 K, and can undergo 10 cycles upon alternative UV and visible light
irradiation without obvious decay (Figs. S21-S23 in Supporting information). Such results revealed that ap-YL underwent reversibly
and efficiently on-to-off fluorescent switching upon light irradiation with excellent thermal bistability and fatigue resistance.
Then we checked the photochromic behavior of polymer PYL in solution (Fig. 2b, Table S1, Figs. S21-S23). As depicted in Fig. 2b,
the absorption and emission curve of polymer PYL is similar to that of its photochromic monomer. Upon UV irradiation, the THF
solution of PYL turned from colorless to red along with a new broad absorption band centered at 554 nm, indicative of the
transformation from open to closed isomer. Meanwhile, its initial emission centered at 561 nm gradually quenched with red-shifted
wavelength. Until reaching the photostationary state, the photocyclization conversion ratio was up to 95%. Similar to monomer YL,
PYL displayed outstanding thermal stability and fatigue resistance (Figs. S21-S23). It can be concluded that after polymerization, the
polymer still possesses excellent photoresponsive properties with on-to-off fluorescent switching in solution.
Traditional fluorescent photochromic system always faces the problem of the aggregation-caused quenching, which greatly limited
its practical application. Thus we further checked the fluorescent properties of both monomer YL and polymer PYL at aggregate state
(Figs. 2c and d, Figs. S24 and S25 and Table S2 in Supporting information).
As depicted in Fig. 2c, the emission spectra of open isomers ap-YL and ap-PYL in THF/H2O mixtures with different water fraction
were measured to investigate their AIE behavior. ap-YL showed weak emission in pure THF with a fluorescence quantum yield of
1.04% (Fig. 2c, Table S2). Then with the water fraction increasing from 0 to 60%, the emission gradually decreased accompanied by
red-shifted wavelength. Such phenomenon can be ascribed to the D-π-A structure of the open isomer, which consists of strong electron
acceptor (central benzobisthiadiazole bridge) and strong electron donor (side aryl group) [40-42]. Then the fluorescence restored
obviously while water fraction up to 80%, reaching maximal emission efficiency of 3.7%. As for polymer PYL, its open form
displayed similar fluorescent behavior in THF/H2O mixtures to the monomer ap-YL. Its emission gradually quenched along with the
addition of water into THF, then the fluorescence restored while water fraction over 60%. During this process, its emission efficiency
changed from 1.42% in THF to 3.8% in water (Table S2).
As for the closed isomers, c-YL displayed weak emission in pure THF. And then upon the addition of water, its emission increased
sharply accompanied with fluorescence quantum yield from 0.12% to 11.2% (Fig. 2c, Table S2). Then considering the difficulty in
obtaining the closed isomer of PYL and the high conversion ratio (95%) of photocyclization reaction, we observed the fluorescent
properties of pss-PYL in THF/H2O mixtures instead (Fig. 2d). Similar to monomer c-YL, pss-PYL displayed excellent AIE
characteristic with the fluorescent quantum yield from 0.44% to 10.5% (Table S2).

Fig. 2. Photochromic and AIE properties of monomer ap-YL and polymer PYL. Absorption and fluorescence spectra of (a) monomer ap-YL (2.0  10-5 mol/L)
and (b) polymer PYL (1.80 g/L) in THF upon UV irradiation at 313 ± 10 nm. Excitation for fluorescence of monomer and polymer is set at isobestic point at
341 and 343 nm, respectively. Inset images in (a) and (b) show the color and emission changes of monomer and polymer, respectively. (c) Relative fluorescence
intensity I/I0 plots of open isomers ap-YL (2.0  10-5 mol/L) and ap-PYL (1.0 g/L) in THF/H2O mixtures with different water ratios, excited by 341 and 343 nm,
respectively. (d) Relative fluorescence intensity I/I0 plots of closed isomers c-YL (2.0  10-5 mol/L) and pss-PYL (1.0 g/L) in THF/H2O mixtures with different
water ratios, excited by 552 and 554 nm, respectively. Here, I is the fluorescence intensity with different water ratios, and I0 is initial fluorescence intensity in
pure THF.
The excellent AIE properties of both YL and PYL motivated us to further explore their photoswitching fluorescence in solid state.
Firstly, given that the powder of YL failed to undergo photocyclization reaction, the fluorescent switching upon light irradiation was
tested in aggregated state (THF/H2O = 2/8, v/v, Fig. S26 in Supporting information). Upon visible light irradiation (λ > 510 nm), the
initial strong emission of c-YL gradually decreased (Fig. S26a), indicating that c-YL in aggregated state can undergo
photocycloreversion reaction to the open form. However, after that, with continuous UV irradiation, the emission appeared negligible
changes (Fig. S26b), indicating that the open isomer failed to undergo photocyclization reaction, consistent with the photoinduced
performance of YL powder. This result revealed that monomer YL can only undergo irreversible light-driven fluorescent switching
upon aggregation. Such behavior may be attributed to that the rigid environment upon aggregation greatly limited the intermolecular
space, thus failing to afford the photocyclization process (Fig. 1) [30,35].
In contrast, it was discovered that PYL powder displayed reversible photoswitching under alternative UV and visible light
irradiation (Fig. 3a and Fig. S25). Before UV exposure, the color of PYL powder was pale yellow with a broad emission band around
500-600 nm. Then under irradiation by a hand-held ultraviolet lamp (λ = 365 ± 20 nm), the color gradually changed to dark red along
with red-shifted emission (641 nm), corroborating the photo-induced conversion from open to closed isomer. During this process, its
emission switched with a high contrast ratio (fluorescence efficiency increased from 6.1% to 24.0%). Then continuous visible light
irradiation (λ > 510 nm) can drive the red powder convert back to the initial pale color state. Such reversible photoswitching of
polymer PYL, different from that of monomer, can be ascribed to that after polymerization, the environment around the photochromic
moiety became softer, thereby bringing enough space to afford efficient photocyclization reaction.
Accordingly, given that the closed isomer of polymer PYL showed strong red emission in solid state, we further investigated the
photoinduced fluorescent changes of PYL film, expecting to achieve turn-on fluorescent switching (Fig. 3). Here, the polymer film was
prepared by coating THF solution with polymer PYL (30 wt%) on the top of quartz glass under room temperature, with a thickness of
1.05 μm (Fig. S27 in Supporting information). Firstly, the color of this film was pale yellow, along with negligible emission excited by
554 nm due to negligible absorption (Fig. 3b). Then upon UV (λ = 365 ± 20 nm) exposure, the pale yellow color of PYL film turned
red immediately. Simultaneously, its emission displayed obvious enhancement, which can be ascribed to the conversion from open to
closed isomer. After that, under visible light (λ > 510 nm) exposure, such emission centered at 645 nm gradually decreased and finally

converted back to the initial state, accompanied by the obvious color changing from bright red to pale yellow (Fig. 3c). Notably, the
contrast ratio of fluorescent signal was up to 177 during this cycle. And upon alternative UV and visible light exposure, little decay
occured even after 11 cycles (Fig. 3d). SEM photos also indicated that there were no obvious defects before and after light exposure for
PYL film (Fig. S28 in Supporting information). Accordingly, the polymer PYL film exhibited efficiently and reversibly off-to-on
fluorescent switching upon light irradiation with excellent fatigue resistance and high signal contrast.
Advances in thin-film technology have enabled a wide range of breakthroughs in the areas of flexible display, optical coating, and
energy storage. Our polymer film material PYL exhibits excellent photoresponsive luminescence and AIE performance which was
further demonstrated in anti-counterfeiting (for the detailed preparation procedure, see the Supporting Information). As shown in Fig. 4,
the number “100” was readily coated on the blank area of a banknote by THF solution with polymer PYL (10 wt%), and the coated
area was almost invisible under room light. While upon UV light (λ = 365 ± 20 nm) irradiation, the red number “100” quickly appeared
in the coated area within 0.1 s, accompanied with bright red emission, thus the number turning to visible. Notably, such strong
emission can be clearly observed under both room light and UV light. Then, under visible light (λ > 510 nm) irradiation, the emission
and color of “100” gradually faded out and reverted to the initial invisible state. Also, the polymer PYL can be well attached to wine
bottle. Similarly, the emblem figure coated on the bottle quickly changed from invisible to red with clear outline upon UV irradiation,
along with bright red emission, and then returned to the invisible state under visible light exposure. Such photocontrollable
performance between invisible and visible state successfully verified the application of polymer PYL in anti-counterfeiting.
Fig. 3. Fluorescent switching of polymer PYL in solid powder and film. (a) Room light (up) and fluorescent (down) pictures of PYL powder under UV (λ = 365
± 20 nm) and visible light (λ > 510 nm) irradiation, in which the fluorescent pictures were recorded under UV light (λ = 365 ± 20 nm). Fluorescence spectra of
PYL film upon (b) UV light (λ = 365 ± 20 nm; 0, 0.5, 1.5, 3.5, 6.5, and 10 s) and (c) visible light (λ > 510 nm; 0, 5, 15, 45, 90, and 150 s), excited at 554 nm.
Inset images showed the color changes of film before and after UV irradiation. (d) Fatigue resistance of fluorescence intensity at 645 nm upon alternative UV (λ
= 365 ± 20 nm) and visible light (λ > 510 nm) irradiation for PYL film.
Fig. 4. Application of photoresponsive AIE polymer film in anti-counterfeiting. The photoinduced color and emission changes between invisible and visible
transformation on a banknote (up) and wine bottle (down) coated with PYL. The color pictures were recorded under room light, and the fluorescent pictures
were recorded under UV light (λ = 365 ± 20 nm).
In this work, a novel photocontrollable fluorescent polymer PYL with excellent AIE activity is constructed via encoding steric-
hindrance photochromic fluorophore YL into flexible polymer chains. After polymerization, polymer PYL possesses excellent
photoresposive properties and AIE activity inherit from monomer YL, and meanwhile successfully provides requisite free space for
efficient photocyclization reaction in aggregate state. In solution, both the open and closed isomer of PYL exhibit weak emission. In

contrast, the open isomer of PYL shows weak color and fluorescence while its closed isomer exhibits bright red color and fluorescent
emission in solid state. Upon alternative UV and visible light irradiation, both its powder and film successfully realize reversibly off-
to-on photoresponsive color and fluorescent switching with quick response and high contrast. Such fascinating photoresponsive
polymer film are well coated on banknote and wine bottle, exhibiting reversibly invisible/visible both color and fluorescent information,
indicative of its promising application in photo controllable anti-counterfeiting.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could
have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China for Science Center Program (No.
21788102), Creative Research Groups (No. 21421004) and Key Project (No. 21636002), Shanghai Pujiang Program (No.
20PJ1402900), and National key Research and Development Program (No. 2016YFA0200300), Shanghai Municipal Science and
Technology Major Project (No. 2018SHZDZX03), and Program of Introducing Talents of Discipline to Universities (No. B16017).
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