Bioinspired Synergistic Photochromic Luminescence and Programmable Liquid Crystal Actuators

Article abstract of DOI:10.1002/anie.202101881

Bioinspired smart materials with synergistic allochroic luminescence and complex deformation are expected to play an important role in many areas of science and technology. However, it is still challenging to fabricate such soft actuators with high programmability that can be manipulated in situ with high spatial resolution. Herein, we have incorporated terminally functionalized aggregation-induced emission active tetraphenylethene derivative and photochromic spiropyran moieties into the networks of liquid crystal elastomers through covalent bonding to obtain the synergistic photochromic luminescence and programmable soft actuators. Bio-mimic functions and light-induced auxetic metamaterial-like devices were shown to be feasible based on the combination of assembly and origami-programming strategy. These bioinspired devices with synergistic photochromic luminescence and complex photodeformation abilities provide an elegant strategy to design multi-functional liquid crystal actuators.

Full text of DOI:10.1002/anie.202101881

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 11247–11251
International Edition: doi.org/10.1002/anie.202101881
German Edition: doi.org/10.1002/ange.202101881
Actuators Very Important Paper
Bioinspired Synergistic Photochromic Luminescence and
Programmable Liquid Crystal Actuators
Yinliang Huang, Hari Krishna Bisoyi, Shuai Huang,* Meng Wang, Xu-Man Chen, Zhiyang Liu,
Hong Yang,* and Quan Li*
Abstract: Bioinspired smart materials with synergistic allo-
chroic luminescence and complex deformation are expected to
play an important role in many areas of science and
technology. However, it is still challenging to fabricate such
soft actuators with high programmability that can be manip-
ulated in situ with high spatial resolution. Herein, we have
incorporated terminally functionalized aggregation-induced
emission active tetraphenylethene derivative and photochromic
spiropyran moieties into the networks of liquid crystal
elastomers through covalent bonding to obtain the synergistic
photochromic luminescence and programmable soft actuators.
Bio-mimic functions and light-induced auxetic metamaterial-
like devices were shown to be feasible based on the combina-
tion of assembly and origami-programming strategy. These
bioinspired devices with synergistic photochromic lumines-
cence and complex photodeformation abilities provide an
elegant strategy to design multi-functional liquid crystal
actuators.
fluorescence and the abilities to simultaneously alter their
shapes, have been more attractive,[3] because of the potential
to camouflage, intimidate, and communicate in darkness such
as Aequorea victoria.[4] Chen et al. reported a pH-responsive
bilayer hydrogel actuator which showed synergetic shape-
morphing and fluorescent color changing behaviors. How-
ever, this hydrogel actuator only emitted one fluorescence
color, while diverse colors are required to satisfy the survival
needs of creatures in nature.[5] In order to diversify the
fluorescent colors, they utilized the lanthanide-ion (Eu3+ and
Tb3+) coordination to obtain a multicolor fluorescent bilayer
hydrogel actuator.[6] Furthermore, Tang et al. made use of an
aggregation-induced emission (AIE) molecule tetra-(4-pyr-
idylphenyl)ethylene to fabricate the pH-responsive deform-
able hydrogel actuators with the abilities to simultaneously
tune their fluorescence colors and brightness.[7] Although
synergetic control over the fluorescent colors and shapes in
hydrogels has been realized, unfortunately, there are still
limitations in those previously reported systems: 1) Variable
aqueous environments are required when the hydrogels are
actuated, in which situation the contact operating mode limits
their applications;[8] 2) the deformation rates are relatively
slow, and color-switching is not convenient because of the
difficulty in altering ions and pH;[9] 3) the deformation of
hydrogel actuators is simplex and difficult to be in situ
manipulated with high spatial resolution.[9c]
The development of liquid crystal (LC) actuators provides
an inspiration toward overcoming many of the above
disadvantages. Based on the manipulatable and program-
mable mesogenic orientation, three-dimensional (3D)
motions and accurate remote control over the movements
can be facilely realized.[10] Additionally, the deformation,
induced either by phase transition or alignment reforming of
LCs, is fast and large under external stimulus.[9a,11] The above
strengths have enabled LC actuators for employment in soft
robotics.[12] Therefore, realization of synergistical control over
the 3D deformation and allochroic fluorescence in LC
actuators is very important for endowing soft robotics with
more biomimetic functions.
I
n nature, many living organisms are capable of synergisti-
cally altering their shapes and colors so as to camouflage,
communicate, or intimidate for surviving in their environ-
ment.[1] For example, cephalopods simultaneously change
their appearance to blend with the surroundings, when
hunting or to evade predators.[1a] A frillneck lizard will
unfurl its frills up to six times the width of its head, and
simultaneously turn the frill color to red-orange, for the
purpose of regulating body temperatures and intimidate
predators/rivals.[2]
These marvelous biological behaviors have inspired
researchers to develop smart adaptive materials which are
able to tune their colors and shapes according to environ-
mental changes. So far, smart hydrogels with allochroic
[*] Y. Huang, Dr. S. Huang, Dr. M. Wang, Dr. X. Chen, Dr. Z. Liu,
Prof. H. Yang, Prof. Q. Li
Institute of Advanced Materials, School of Chemistry and Chemical
Engineering, and
Jiangsu Province Hi-Tech Key Laboratory for Bio-medical Research
Southeast University, Nanjing 211189 (China)
E-mail: huangshuai1991@seu.edu.cn
yangh@seu.edu.cn
In this study, we have fabricated a smart soft actuator by
incorporating tetraphenylethene (TPE) and spiropyran (SP)
moieties as the tunable fluorophores into the LC elastomer
(LCE) matrix. Here, TPE moiety served as an archetypical
AIE molecule, providing strong fluorescent emission.[13] The
photochromic SP moiety could reversibly isomerize between
the nonfluorescent closed form and the red fluorescent open
merocyanine (MC) form under sequential irradiation with
UV and visible light.[14] As a result, photochromic lumines-
cence could be realized via the combination of functionalized
quanli3273@gmail.com
Dr. H. K. Bisoyi, Prof. Q. Li
Advanced Materials and Liquid Crystal Institute and
Chemical Physics Interdisciplinary Program
Kent State University, Kent, OH 44242 (USA)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202101881.
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tion and fluorescence spectroscopy. Here, we chose LCE1 as
a case study. The tested samples were prepared by spinning-
coating the mixture of the components for LCE1 onto glass
substrates, and then curing them at 508C for 48 h. As shown in
Figure 1a the absorption at 575 nm, attributed to the charac-
teristic peak of MC (open form), appeared, and the intensity
gradually increased, when the sample was irradiated with UV
Scheme 1. A) Illustration of the chemical structure of TPE-ene and the
reversible conversion between the open form MC (red fluorescent) and
the close form SP (nonfluorescent) of a functionalized spiropyran.
B) Illustration of the mechanism of simultaneously controlling the
fluorescence changes and deformation. The fluorescence changes
because of the isomerization of SP, while the deformation is attributed
to the thermal induced phase transition of LCs in the polymer
networks.
Figure 1. Time-dependent a) UV/Vis absorption spectra and b) fluores-
cence spectra response of LCE1 film (lex =365 nm) with UV light
(l=365 nm). c) Photo-fatigue resistance of LCE1 film upon irradia-
tions of UV light (l=365 nm) for 150 s and visible light (l=530 nm)
for 90 s alternatively. d) Fluorescence spectra response of LCE1 film
with different temperatures.
TPE and SP derivatives, as illustrated in Scheme 1A.
Furthermore, the motions of LCEs were programmed by
manipulating the alignment of mesogens and triggered by
near-infrared (NIR) light.[12] Based on the above designs,
synergic control over the 3D shape-morphing along with
allochroic luminescence was enabled, as shown in Scheme 1B.
Herein, UV and visible light sources were exploited to tune
the fluorescence colors, while the deformation of the LCE
film was controlled by a NIR light source, indicating that the
whole system could be manipulated in a precise and non-
contact way.[15,16] Moreover, we realized the camouflaging and
crawling behaviors mimicking discoloration of a caterpillar.
By exploiting the programmability of the materials, we
demonstrated that the fluorescence changeable LC actuators
hold wide potentials in various applications including bio-
mimetic soft robotics, camouflage, metamaterials, and signal
transmission devices.
To systematically investigate the synergic deformation
and fluorescence behaviors, different components were
imposed in LCE0, LCE1 and LCE2, as listed in the Support-
ing Information Table S1 and Figure S2. Briefly, LCE0 was
the uniaxially-stretched LCE film doped with photothermal
dye YHD796. LCE1 was the polydomain film without
YHD796, but contained the covalently bonded fluorescent
moieties (SP-2ene and TPE-ene). In LCE2 which contains
both YHD796 and fluorescent dyes, origami was imple-
mented to obtain the programmed hybrid mesogenic align-
ments. The characterization of the mesomorphic properties is
presented in the Supporting Information, Figures S3 and S4.
The photoresponsive behaviors of the LCE actuators were
investigated in detail by ultraviolet-visible (UV/Vis) absorp-
(365 nm) light. After irradiated for 120 s, it reached the
photostationary state. Figure 1b showed the fluorescence
spectra of LCE1 with the excitation wavelength at 365 nm
under different UV exposure time. The fluorescent intensity
of TPE at 487 nm (blue–green fluorescence) gradually
decreased under UV irradiation, while the intensity of the
peak at 655 nm ascribed to the emission of MC increased.
These results proved the occurrence of isomerization from the
closed form SP (nonfluorercent) to the open MC form (red
fluorescent) in the LCE1 film. Additionally, since the
absorption peak of MC and the fluorescent emission peak
of TPE overlapped partially, the energy emitted by TPE were
partially absorbed by MC, resulting in a significant decrease
in the fluorescence intensity of TPE at 487 nm, demonstrating
a Forster resonance energy transfer (FRET) type process, as
illustrated in the Supporting Information, Figure S6.[17] Fig-
ure S7a showed that the absorption peak at 575 nm disap-
peared under visible light (530 nm) irradiation for 90 s,
proving the reversible conversion between MC and SP. To
investigate the photo-fatigue resistant properties of the LCEs,
the sample was irradiated in sequence with UV (150 s) and
visible light (90 s). The ratios of the fluorescent intensities
(I655nm/I487nm) remained unchanged within 6 cycles, indicating
that the LCEs possessed good photo-fatigue resistant proper-
ties, as shown in Figure 1c.
To verify the thermo-responsive behaviors, fluorescence
spectra of LCE1 at varying temperatures were tested. As
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shown in Figure 1d and the Supporting Information, Fig-
then returned to the original shape as the light was removed.
ure S7b, the variable UV/Vis absorption spectra of LCE1 on
heating were similar to those under UV irradiation, because
the ring-opening reaction of SP also occurred upon heating.
As shown in Figure 1d, the fluorescent intensity of LCE1 at
655 nm first increased and then decreased as the temperature
gradually rose because of the transformation from the close
form SP (nonfluorescent) to the open form MC (red
fluorescent). Moreover, the nematic-to-isotropic transition
of LCs also affected the stacking of molecules, leading to
quenching of the fluorescence emission.[18] Furthermore, the
photoresponse of LCE2 was also investigated by UV/Vis
absorption and fluorescence spectroscopy (Supporting Infor-
mation, Figures S8, S9). The spectra were similar to those of
LCE1, indicating that the NIR dye YHD796 did not adversely
interfere the light-regulation of multicolor fluorescence
interconversion.
The actuators were further utilized to mimic biological
functions. In nature, caterpillars could wriggle to navigate
accompanied by adjusting their colors to camouflage and
communicate. Figure 2a illustrates the crawling and color
changing process of a caterpillar. Inspired by such biological
functions, an LCE-based soft actuator was designed to imitate
the movement of caterpillars. The soft actuator was a bilayer
LCE film manufactured by attaching two monoliths together,
in which the upper layer was the polydomain LCE1 with
photochromic luminescence, while the bottom layer was the
monodomain LCE0 with the NIR-driven actuating proper-
ties. Therefore, the obtained soft actuator can imitate the
crawling of a caterpillar under control of an NIR light source
(808 nm, 0.19 WcmÀ2). When the “body” of the caterpillar
was exposed to NIR light, it rapidly bended downward, and
The bending-unbending deformation would drive the actua-
tor wriggling forward like a caterpillar because the exposed
body part would obtain a bigger friction than that of the
unexposed part according to Amontonsꢀ law.[19] Repeating the
on/off cycles of NIR light would drive the bionic actuator to
crawl forward rhythmically. The detailed process of the
caterpillar-mimicking motions was recorded in the Support-
ing Information Figures S10, S11.
Color changing behaviors mimicking a caterpillar were
realized in the obtained complex soft actuator.[20] The
actuator crawled 1.1 cm driven in five on/off cycles irradiation
of NIR light, as shown in the Supporting Information,
Figure S12a1,a2. The emission color could be photo-manip-
ulated. As shown in the Supporting Information, Fig-
ure S12a3–a6, the actuator was preprocessed with UV light
to give the purple red initial state, where the grafted SP
moiety was kept as the open form MC. The color remained
unchanged on exposure to NIR light. After turning off the
NIR light (808 nm) and alternately turning on visible light
(530 nm) and UV light (365 nm), it was found that the
caterpillarꢀs color interconverted between light-yellow and
purple red. Moreover, the allochroic luminescence behaviors
enabled the artificial caterpillar to work in night environ-
ments (Figure 2b). On exposure to 365 nm UV light as the
fluorescence exciting light, the actuator emitted red fluores-
cence. Spontaneously exerting the NIR irradiation could
direct the caterpillar-like fluorescent actuator to crawl
forward (Figure 2bi–iii). Turning off all the light sources
made the actuator nonluminous and hide in darkness (Fig-
ure 2biv). Following the subsequent irradiation of a visible
light (530 nm) for 90 s which caused the MC-SP isomer-
ization, the exciting light (365 nm) was turned on again to
excite the blue-green fluorescence. Therefore, the fluorescent
color changed to blue-green and the actuator camouflaged
itself in the blue-green environment (Figure 2bv–vi). There-
after, recovering of the photochromic SP led to the gradual
change of the fluorescent colors from blue-green to red under
continuous 365 nm UV light irradiation (Figure 2bviii and
bxiv). More importantly, the actuator could navigate in
darkness manipulated wirelessly by NIR light (808 nm), and
suddenly appeared in another place by emitting its fluores-
cence in response to UV excitation, which is very meaningful
for creature predating or evade predators in especially serious
environments (Figure 2bviii–xi). After all the activities have
been done, the actuator crawled and navigated to the initial
location, followed by camouflaging itself again and hiding in
darkness to wait for the next command (Figure 2bxi–xiii).
These results prove that the synergic photo-manipulated
motions and photochromic luminescence behaviors of the
well-designed LCE actuator could be utilized to devise
biomimetic devices toward environment-adaptable soft
robotics with the capability to perform complex tasks in
special conditions.
Considering that LCEs with designed local mesogenic
alignments could perform sophisticated motions, we pro-
grammed an LCE film (LCE2) with hybrid local alignment by
folding methods. Mesogens at the crease were oriented by the
gradient-distributed mechanical stress. Therefore, a multifunc-
Figure 2. a) Photo images of crawling and camouflaging behaviors of
caterpillar. b) The biomimetic crawling and camouflage process of the
bilayer LC soft actuator manipulated by light. Note: the blue, green
and red backgrounds were created artificially by painting with fluores-
cent dyes.
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tional 3D LC soft actuator (“paper garland”) was created by
manually folding and cutting, which was also named as
kirigami art,[21] as shown in Figure 3a. When exposed to
incident light with different wavelengths, the actuator (“paper
garland”) reversibly deformed in a 3D manner along with
fluorescence color changing (Supporting Information, Fig-
ure S13). These results prove various potentials of the
materials to be freely processed and programmed for more
sophisticated applications.
The LC actuator with photochromic luminescence can be
exploited to mimic more sophisticated bio-functions, which is
actually more important for creatures to survive various
environments. The discoloration-accompanied furling–
unfurling behavior of frillneck lizardsꢀ frills gives a typical
example.[2] Figure 4a shows the illustration of synergic
motion-luminescence variation that mimicked the bio-func-
tions of frillneck lizardsꢀ frills. Based on the aforementioned
Figure 4. a) Photo images of frillneck lizards that spread out or fold
their frills. b) Synergetic color-changing and shape-morphing behaviors
(spread out or folded state) of biomimetic frills (frillneck lizards).
Figure 3. Simultaneous multicolor fluorescence change and complex
shape-morphing of 3D liquid crystal soft actuator (LCE2): a) “paper
garland” and b) “Miura fold” under light (UV light/visible light/NIR
light) stimulation.
“paper garland” and “Miura fold” strategy, we created
a complex 3D LC actuator to mimic this behavior as shown
in Figure 4b and the Supporting Information, Figure S17.
One LCE film (LCE2) was first folded in the “Yoshimura”
way, thus occupying very little space. It emitted red fluores-
cence under continuous UV irradiation. When exposed to
NIR light, it suddenly unfurled and the fluorescent color
changed from bright red to dull red. Turning off NIR light
caused a synchronous restoring of the luminescent color and
shape to the original state. Removing the UV light terminated
the luminescence of the actuator which could then hide into
the dark environment. Whereafter, the fluorescence reap-
peared but the color changed to blue–green upon visible light
(530 nm) irradiation for 90 s. Re-infliction of NIR along with
UV light drove the folded actuator to unfurl again accom-
panied with fluorescence changing to dull red. Continuous
UV irradiation without other light sources could turn the
fluorescent colors back to red. Detail information can be
found in the Supporting Information, Figure S17 and
Movie S2. The results demonstrate the potentials of LC
actuators containing both LC units and photoresponsive AIE
components in realizing sophisticated synergy of photo-
deformation and photochromic luminescence, which provides
a new strategy for developing biomimetic soft robotics that
are expected to be of great value to various scientific areas.
In summary, we have fabricated the photochromic lumi-
nescent LCE actuators using covalently linked TPE and SP
moieties as the functional fluorescent groups. We have
demonstrated that the LCE based actuator could be exploited
to mimic biological functions, such as the crawling and
discoloration behaviors of caterpillar in day/night environ-
ments, and the photoresponsive fluorescence and shape-
morphing behaviors of other organisms. In addition, sophis-
ticated synergy modes of the photo-driven movements and
Auxetic mechanical metamaterials (negative Poissonꢀs
ratio metamaterials) expand transversally when it is stretched
along the axial, which possess good shear resistance and
negative thermal expansion properties. These fantastic prop-
erties endow the material with a wide range of applications.[22]
It has been demonstrated that the negative Poissonꢀs ratio
materials could be constructed by Miura folding origami.[23]
Hence, we fabricated one LC actuator with NIR-induced
negative Poissonꢀs ratios (according to the Poissonꢀs ratio
formula,[24] the Poissonꢀs ratio of the actuator was calculated
as À1.85 (Supporting Information, Figure S14)), which pos-
sessed the light-induced two-way shape memory with large
deformation, as shown in Figure 3b. Additionally, the pro-
grammed LC actuator was capable of performing reversible
fluorescence color changing and auxetic-like behaviors under
the modulation of lights with different wavelengths (Support-
ing Information, Figures S15, S16 and Movie S1). Specifically,
NIR light would drive the Miura-folded actuator to expand in
all directions, accompanied by gradually weakening the red
fluorescence. The fluorescent color and shape would syn-
chronously restore to the original state on removal of NIR
irradiation. Moreover, the fluorescent color changed to blue-
green after visible light (530 nm) irradiation, and then
changed to dull red along with the auxetic-like deformation
on exposure to NIR. The reversible process indicates that
such materials are potentially applicable in mechanical
metamaterials with modulable optical properties, which are
expected to be applied in biomimetic soft robotics with the
abilities to camouflage and transmitting information.
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proved to be quite feasible in this work, which shows great
potential in obtaining multiple nature-inspired functions,
which could be incorporated into one device for multifunc-
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from National Natural Science Foundation of China (No.
21971037, 52003050, 22001035, 51903048) and Jiangsu Pro-
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Conflict of interest
The authors declare no conflict of interest.
Keywords: bioinspired function · complex programmability ·
liquid crystal actuator · photochromic luminescence
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Manuscript received: February 5, 2021
Accepted manuscript online: March 2, 2021
Version of record online: April 8, 2021
Angew. Chem. Int. Ed. 2021, 60, 11247 –11251
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