Programmable Chromism and Photoluminescence of Spiropyran-Based Liquid Crystalline Polymer with Tunable Glass Transition Temperature

Article abstract of DOI:10.1002/anie.202107048

Spiropyran-based materials (SPBMs) can give responses to the stimulations induced by the light, heat, force, or pH, which have been used as triggers for many smart materials. Here, a cross-linkable SPBM containing mesogenic-units is synthesized, which is pale-colored, non-photoluminescent and non-mesogenic at a spiro form, but dark-colored, photoluminescent, and mesogenic at a merocyanine form. Moreover, the dynamic interconversion behavior of the form in the different chemical environments are distinct. Liquid crystalline polymers (LCPs) containing the SPBMs cross-linked via visible light, own a photoswitchable glass transition temperature (T_g) and retain the switchable property; however, the SPBMs cross-linked via UV light will be locked at the MC state, because the molecular movement was frozen at the room temperature lower than the given T_g of the LCP. Thus, programmable chromism and photoluminescence based on the tunable T_g can be endowed to the functional materials prepared from the SPBMs.

Full text of DOI:10.1002/anie.202107048

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How to cite:
International Edition: doi.org/10.1002/anie.202107048
German Edition: doi.org/10.1002/ange.202107048
Responsive Materials
Programmable Chromism and Photoluminescence of Spiropyran-
Based Liquid Crystalline Polymer with Tunable Glass Transition
Temperature
Wei Hu, Chang Sun, Yunxiao Ren, Shengyu Qin, Yu Shao, Lanying Zhang, Yu Wu,*
Qian Wang,* Huai Yang,* and Dengke Yang
[
1]
Abstract: Spiropyran-based materials (SPBMs) can give
responses to the stimulations induced by the light, heat, force,
or pH, which have been used as triggers for many smart
materials. Here, a cross-linkable SPBM containing mesogenic-
units is synthesized, which is pale-colored, non-photolumines-
cent and non-mesogenic at a spiro form, but dark-colored,
photoluminescent, and mesogenic at a merocyanine form.
Moreover, the dynamic interconversion behavior of the form in
the different chemical environments are distinct. Liquid
crystalline polymers (LCPs) containing the SPBMs cross-
linked via visible light, own a photoswitchable glass transition
robotics, and high-resolution imaging system. Spiropyran-
based molecule (SPBM) early was concerned as one of the
thermochromic materials,
[
2]
which can convert between
a colorless spiro (SP) and a colored merocyanine (MC)
[
3]
form. Subsequently, its photochromic property caused more
intensive attention and investigations, due to their high
potential for applications to optically rewritable data storage,
optical switching, and chemical sensing. Liquid-crystalline
polymer (LCP) materials as a novel kind of soft material
usually were used to be the matrixes of many advanced
functional materials, due to their unique molecular anisotro-
py, good formability, and excellent mechanical properties. It
was promising to achieve new smart materials with some
potential functions by introducing a tailored SPBM to LCPs.
Researchers achieved rewritable optical storage via doping or
cross-linking directly primitive SPBMs into LCs, by which the
respective characters of spiropyran and mesogens were
[
4]
[
5]
temperature (T ) and retain the switchable property; however,
g
the SPBMs cross-linked via UV light will be locked at the MC
state, because the molecular movement was frozen at the room
temperature lower than the given Tg of the LCP. Thus,
programmable chromism and photoluminescence based on
the tunable T can be endowed to the functional materials
g
[
4a,f,6]
prepared from the SPBMs.
express together.
Some effective strategies combing
SPBM with other functional materials to achieve program-
[
7]
Introduction
mable chromism or deformation also were reported. In this
work, to enhance the molecular anchoring effects between the
[8]
Functional materials based on molecular switches are able
to achieve reversible changes of the external properties such
as color, shape, and wettability, etc., which show great
potentials in the fields of sensor, actuator, data storage, soft
SPBMs and the LC molecules, designing and synthesizing
cross-linkable a spiropyran-based molecule containing meso-
genic-units as the trigger of the LC smart material was
performed.
[
*] W. Hu, S. Qin, Y. Shao, L. Zhang, Q. Wang, H. Yang
Beijing Advanced Innovation Center for Materials Genome Engi-
neering & Department of Materials Science and Engineering College
of Engineering, Peking University
Results and Discussion
Performances of the Designed SPBM in Different Chemical
Environments
Beijing 100871 (P. R. China)
E-mail: 1116qianwang@pku.edu.cn
yanghuai@pku.edu.cn
The synthesis routes for the designed SPBM are shown in
Scheme 1 and the detailed syntheses and characterization
data are shown in supporting information. At the SP form,
two mesogenic-units are linked by a spiro and restrained to
a twisty “V” letter configuration, called C6A-NO2-SP (Fig-
ure 1A). The intermediate spiropyran unit can convert to
a planar merocyanine structure from a ring-closed spiropyran
C. Sun, Y. Ren
School of Materials Science and Engineering, University of Science
and Technology Beijing
Beijing 100083 (P. R. China)
Y. Wu
Department of Obstetrics and Gynecology, Peking University Third
Hospital
Beijing 100191 (P. R. China)
[2,3]
form as the spiro carbon-oxygen (CÀO) bond rupturing.
E-mail: pheonix1997@163.com
When the spiro CÀO bond ruptures and the restrained V-
shaped configuration releases to a rodlike MC form called
C6A-NO2-MC (Figure 1B) according with a classical config-
uration of the LCs.
D. Yang
Chemical Physics Interdisciplinary Program in Liquid Crystal Insti-
tute, Kent State University
Kent, OH 44242 (USA)
The synthesized pale-yellow solid SPBM was sandwiched
by two glass substrates and placed on a hot stage, which
turned to be blue-violet with an increasing temperature until
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.202107048.
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operations are vain. Dynamic X-Ray Diffraction (XRD)
patterns upon the variable temperature of the SPBM, which
has the same thermal history as round 1 in the DSC
measurement, are shown in Figure 2B (The 2D images of
the XRD measures are shown in Supplementary Figure S3).
Serials of diffraction peaks appear at the range of little q value
after the temperature arising to 508C, and these peaks turn to
be sharper with the temperature increasing until disappear
after clearing. It demonstrates that the order of the molecular
arrangement increases with the increasing temperature be-
[
9]
fore clearing. Rodlike configuration is more beneficial to
form a regular molecular arrangement than V-shaped con-
figuration, therefore, C6A-NO2-MC is crystallographic but
C6A-NO2-SP is amorphous. Combining the results of XRD
and DSC measurement, the exothermic peak can be attrib-
utable to the exothermic ring-opened reaction of C6A-NO2-
SP to C6A-NO2-MC. Moreover, the phase transformation
canꢀt be detected in the DSC measurement upon cooling,
which can attribute to the effect of supercooling and the
[
10]
disturbance induced by the interconversion.
The transition point from nematic to isotropic of 4-Cyano-
4’-n-pentylbiphenyl (5CB), and an LC mixture (5.0 wt%
C6A-NO2-SP in 5CB) detected by polarizing optical micro-
scope (POM) were 35.88C and 34.08C, respectively. The
clearing point of the SPBM obtained from the DSC mea-
surement is approximately 908C, which is much higher than
the 5CB. However, compared with the pure 5CB, the clearing
point of the LC mixture is significantly decline. It demon-
strates that the SPBM at C6A-NO2-SP form is not meso-
[11]
genic. The POM image of the LC mixture almost presents
a completely black state holding at 34.08C (Supplementary
Figure S4a), however, bright LC textures emerge more and
À2
Scheme 1. Synthesis routes for SPBM.
more with the irradiation time of 5.0 mWcm 365 nm UV
light increasing (Supplementary Figure S4b and S4c). There-
fore, this SPBM has switchable mesomorphism that is, it
exhibits non-mesogenic at initial state but mesogenic after
exciting by UV or heating, which is opposite to most stimuli-
responsive molecules such as mesogenic azobenzene and
styrene, etc. derivatives, losing their mesomorphism after an
to be liquid at 808C (Figure 1-i,iii). Subsequently, the blue-
violet liquid slowly returned to be a pale-yellow solid via
staying in dark or irradiating by a bright visible light. And the
pale-yellow solid also can be rapidly urged to a blue-violet
one via UV light irradiation. Therefore, the ring-closed form
C6A-NO2-SP and ring-opened form C6A-NO2-MC can
interconvert via heating or light urging, the schematic of
interconversion of the SPBM is shown in Figure 1C; Fig-
ure 1D and 1E are the photoluminescence and absorption
spectrum of C6A-NO2-MC,respectively. The C6A-NO2-MC
emit a red fluorescence under irradiating by a 365 nm UV
light, the fluorescence peak center at about 630 nm. Its
absorption peak center at about 560 nm, therefore, it shows
blue-violet color under the natural light.
[
5f,12]
isomerization.
In general, to transform the molecular
arrangement from less- to more-ordered state usually is
harder than the process from more- to less-ordered state.
Materials can change their colors via varying the absorp-
tion wavelength in the visible range which depend with their
[
13]
molecular chemical structures. The length of the molecular
conjugation is increased distinctly from C6A-NO2-SP to
C6A-NO2-MC, therefore, an new absorption peak in the
[
2]
visible range emerges. To characterize the variation of
molecular chemical structures, the SPBM was dissolved in
Differential scanning calorimetry (DSC) measurement of
the SPBM indicates that it exhibits a LC state from
approximately 80.08C to 90.08C (Figure 2A, the POM images
of the SPBM are shown in Supplementary Figure S2).
Interestingly, there is an infrequent repeatable exothermic
peak covering 508C to 788C during the heating procedure.
Rounds 1–7 in Figure 2A show the DSC curves of the SPBM
with different thermal histories, it is observed that a holding
operation at 508C either during heating or cooling procedure
can eliminate the odd exothermic peak, but other holding
CDCl3 without any additional operation (Supplementary
À2
Figure S5 ), in CDCl via irradiation of 365 nm 5.0 mWcm
3
UV for 15 min (Supplementary Figure S6) and in [D ]DMSO
6
without any additional operation (Supplementary Figure S7)
to record the 1H NMR spectra, respectively. There is no
distinct difference between the 1H NMR spectra before and
after the irradiation of UV, however, the proportion of the
MC form reaches 30% in DMSO (Figure 2C). That is, the
MC form of the SPBM is steadier in the polar solvent than in
[
14]
the nonpolar solvent.
For convenience, we studied the
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Figure 1. Chemical structures and properties of the SPBM. (A) Chemical structures of C6A-NO2-SP. (B) Chemical structures of C6A-NO2-MC.
C) Schematic of responsive behavior of the SPBM. (D) PL spectra of C6A-NO2-MC. (E) Absorption spectra of C6A-NO2-MC. Inset (i) is the
(
digital photographs of C6A-NO2-SP in a LC cell under the Vis-light; Insets (ii) and (iii) are the digital photographs of C6A-NO2-MC in a LC cell
under the UV-light and the Vis-light, respectively.
changes of the absorption spectra of the SPBM during the
interconversion processes excited by UV and dark at room
temperature (20.08C), respectively. Figure 3A-i illustrates the
RM23/0.5 wt% irgacure 784/0.5 wt% TPO-L. The mixture
was filled into a 50-mm-thick LC cell and irradiated with
À2
a 5.0 mWcm 520 nm green light for 30 min at 408C, after
absorption spectra of
a
solution of the SPBM
opening the cell, a pale-yellow transparent film which
remained the stimuli-responsive properties to UVand heating
was obtained. The absorption spectra of the film were
measured as the preceding operation, changing processes
from the initial state to the PSS (Figure 3C-i) are similar to
that in solution and mesogenic condition; however, the
absorption peak intensity remained 55% of the Max-Abs.
after 6 months in dark and this state almost maintained
stationarily for the following several months (Figure 3C-ii).
Furthermore, the photoluminescence (PL) spectra of the
SPBM upon the above three conditions were measured. The
max excitation wavelength of the SPBM is around 310 nm;
however, the molecule will undergo a ring-opened conversion
to C6A-NO2-MC if it was excited by this short-wavelength
light. To alleviate it, the excitation wavelength was set at
400 nm, the PL spectra of the above three samples before and
À1
À2
(
0.0625 mmolL ) in THF after irradiating by a 5.0 mWcm
3
65 nm UV light for different times. After UV irradiation, an
absorption peak centering at 586 nm raised observably upon
the irradiation time and reached the max absorption (Max-
Abs.) value at a photo stationary state (PSS) for approx-
imately 5 min. Subsequently, the solution was stayed in dark
at the sample location of the spectrograph and measured the
absorption spectra after different times. The intensity of the
absorption peak decreased rapidly to lower than 10% of the
Max-Abs. value after 60 s, then reversed approximately to the
initial state after 90 s (Figure 3A-ii). It demonstrates that the
interconversion between C6A-NO2-SP and C6A-NO2-MC
can occur freely in the solution condition. Subsequently, the
same operation was done to a LC mixture (5.0 wt% C6A-
NO2-SP in 5CB), the absorption spectra are shown in
Figure 3B. The changes of the absorption spectra during the
conversion process from the initial state to the PSS are similar
to that in the solution condition (Figure 3B-i); however, the
reversed speed is significantly slower (Figure 3B-ii). The
process that the absorption peak intensity reduced from the
Max-Abs. to its 10% spent over 5 hours and it canꢀt restore to
the initial state even after over 24 hours, which are far slower
than that in the solution condition.
À2
after a 5.0 mWcm 365 nm light irradiation were obtained
(Figure 3D-i and 3D-ii). The PL spectra of all three samples
are not detectable before the UV irradiation and all greatly
heighten after the UV irradiation, which demonstrates that
C6A-NO2-MC is photoluminescent but C6A-NO2-SP is not.
The measurement values of the PL intensity are negative
related to the speed of reversed conversion from C6A-NO2-
MC to C6A-NO2-SP after the UV irradiation.
To study the performance of the SPBM at a cross-linked
state, it was doped into a polymerizable liquid crystalline
acrylate mixture, whose chemical composition is 5.0 wt%
C6A-NO2-SP/18.0 wt% RM82/25.0 wt% RM105/51.0 wt%
The film obtained via the 520 nm light photopolymeriza-
tion, whose glass-transition temperature (T ) is approximately
g
9.68C (Figure 4A-i), can rapidly interconvert between blue-
violet and pale-yellow via 365 nm and 520 nm light excitation.
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Figure 3. Absorption and photoluminescence spectra of the SPBM in
three different conditions (solution, LC mixture, and film). Absorption
À1
spectra of (A) the solution (0.0625 mmolL SPBM in THF) during the
UV exciting processes (i) and reversing via in dark (ii); (B) the LC
mixture (5.0 wt% SPBM in 5CB) during the UV exciting processes (i)
and reversing via in dark (ii); (C) the LC film (5.0 wt% SPBM in
polyacrylate) during the UV exciting processes (i) and reversing via in
dark (ii). (D) photoluminescence spectra (400 nm excitation) of the
SPBM in three different conditions (solution, LC mixture, and film)
before (i) or after (ii) UV irradiation. The used UV intensities are
À2
5
.0 mWcm , the insets are the corresponding digital photographs
before or after UV irradiation.
À2
The T of this film after a 5.0 mWcm 365 nm UV light
g
irradiation for 2 min increases to 15.28C (Figure 4A-ii); and
for 5 min to 19.68C (Figure 4A-iii). LC textures of the LCP
film during the irradiation were recorded by POM in
reflection mode as shown in Figure S8 and Supplementary
Movie S1, which didnꢀt have distinct change except the
photochromic behavior. The Tg of the SPBM gradually
increases with the UV light irradiation time, therefore, which
Figure 2. Characterization of the SPBM using DSC, XRD, NMR.
(
(
A) The DSC curves of the SPBM upon different thermal histories.
B) Dynamic XRD pattern upon the variable temperature of the SPBM,
which has the same thermal history as round 1 in the DSC measure-
1
ment. (C) Section of High H NMR chemical shifts of the SPBM in
is broadly and finely tunable. The T of some azobenzene-
CDCl and [D ]DMSO, the insets are the corresponding digital photo-
g
3
6
graphs of the samples for the NMR detection.
based materials also can be switched, however, it decreases
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[
15]
with the UV light irradiation.
The interconversion is
repeatable which can be demonstrated by the absorption
spectra during the conversion (Figure 4B). Interestingly,
using a 365 nm UV light instead of the 520 nm green light
to prepare the film, the obtained film (T is approximately
g
2
6.38C, higher than the room temperature, Figure 4A-iv) was
fixed at a blue-violet color state and canꢀt convert to a pale-
À2
yellow state. Irradiating the film with a 5.0 mWcm 520 nm
green light for tens of hours only can slightly dodge the blue-
violet color, and the absorption peak value reduced to
approximately half of the initial value, which can also be
reversed to the initial state by the 365 UV light irradiation
(Figure 4C).
Mechanism Discussion and Models for Application
The dynamic interconversion behavior of the form in the
different chemical environments are distinct. C6A-NO2-SP is
pale-colored, non-photoluminescent and non-mesogenic;
C6A-NO2-MC is dark-colored, photoluminescent and meso-
genic; and they can interconvert upon the excitation of light
or heating. When these SPBMs are dispersed in an organic
solvent, the molecular anchoring effect of molecules is very
weak, so it can rapidly interconvert between the SP form and
the MC form (Figure 5A).
When the SPBMs are dispersed in LCs, initially, the V-
shaped C6A-NO2-SP is not beneficial to form a regular
arrangement with the around LC molecules, so the LC
molecules only can loosely surround C6A-NO2-SP molecules
(Figure 5B-1). The V-shaped C6A-NO2-SP molecules have
enough interspaces to convert to the rodlike C6A-NO2-MC,
Figure 4. T and absorption spectra of the obtained film at different
g
conditions. (A) T of the film (i) obtained via 520 nm photopolymeriza-
g
À2
tion; (ii) obtained via 520 nm photopolymerization after a 5.0 mWcm
65 nm UV light irradiation for 2 min; and (iii) for 5 min; (iv) obtained
3
via 365 nm photopolymerization. (B) Absorption spectra of the film
obtained via 520 nm light photopolymerization, (i) initial state; (ii)
À2
À2
5
5
.0 mWcm 365 nm light irradiation for 5 min; (iii) 5.0 mWcm
20 nm light irradiation for 15 min. (C) Absorption spectra of the film
obtained via 365 nm light photopolymerization, (i) initial state; (ii)
À2
5
3
.0 mWcm 520 nm light irradiation for 18 h; (iii) irradiation by the
À2
Figure 5. Schematic diagram of the mechanism of the stimulus-
responsive behavior of the SPBM in three different conditions. (A) in
the solution; (B) in the LCs; (C) in the film.
65 nm of 5.0 mWcm for 5 min.
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therefore, the mixture rapidly changes to a blue-violet color
and photoluminescent state via the UV irradiation. Mean-
while, a more regular and compact molecular arrangement
occurs in the mixture, because the rodlike C6A-NO2-MC is
beneficial to form a regular arrangement with around LC
molecules with a similar configuration (Figure 5B-2). C6A-
NO2-MC suffer a stronger molecular anchoring effect from
the LC molecules than the solvent molecules, therefore, the
reversed conversion to form a V-shaped configuration must
overcome the restrained-effect induced by the intermolecular
anchoring effect from LC molecules (Figure 5B-3).
When the polymerizable LC acrylate mixture doped with
the SPBM that was kept at SP form (Figure 5C-1), the
mixture was irradiated by 520 nm green light to achieve the in
situ photopolymerization in an LC cell. Thus, the relaxed
molecular arrangement was kept in the LC polyacrylate film
(
Figure 5C-2), the V-shaped C6A-NO2-SP molecules in the
[
16]
film have enough free volume to convert to the rodlike
C6A-NO2-MC. After the configuration conversion to PSS
was completed via the UV irradiation or heating, the T of the
g
film increased from initial 9.68C to 19.68C, indicating that the
molecular arrangement of the film turns to be more regular
and compact (Figure 5C-3). The pale-yellow film changes to
blue-violet color, and this state can be maintained for several
months even years in the dark (The photograph of a pattern
from the photonic printing after different times are shown in
Supplementary Figure S9). Such tardive speed of reversed
conversion may result from the dual-restrained effect induced
by the intermolecular anchoring effect and the stress-strain of
frozen polymer chains at a nearly glassy state. To accelerate
the reversed conversion, irradiating the film via a 520 nm
green light was testified to be effective (Figure 5C-4).
Figure 6. Patterns from thermal and photonic printing. (A) Schematic
diagram of thermal printing. (B) The erasable “butterfly” pattern (Fig-
ure 6B-i) and corresponding PL photograph (Figure 6B-ii) from thermal
printing. (C) Schematic diagram of photonic printing. (D) The ineras-
able “smiling face” pattern (Figure 6D-i) and corresponding PL photo-
graph (Figure 6D-ii) from photonic printing; The inerasable “smiling
face” with the erasable “hairs” pattern (Figure 6D-iii) and correspond-
ing PL photograph (Figure 6D-iv) from photonic printing.
When the polymerizable LC acrylate mixture was doped
with the SPBM that was kept at MC form (Figure 5C-5), the
mixture was irradiated by 365 nm UV light to achieve the in
situ photopolymerization in a LC cell. Thus, the rodlike
configuration of C6A-NO2-MC resulting in a more regular
and compact molecular arrangement of LC molecules, which
[
15]
was confirmed from the higher T of this film, was kept in
g
the LC polyacrylate film; Therefore, the film was locked at
a blue-violet color and photoluminescent state (Figure 5C-6).
Irradiating the film with a 5.0 mWcm 520 nm green light for
photopolymerized via 520 nm green light. Subsequently, the
remained “smiling face” pattern was added some “hairs” via
a 365 nm UV light irradiation with another photomask with
a “hairs” pattern (Figure 6C). Therefore, the “smiling face”
pattern (Figure 6D-i) and corresponding PL pattern (Fig-
ure 6D-ii) are inerasable, however, the “hairs” pattern (Fig-
ure 6D-iii) and corresponding PL pattern (Figure 6D-iv) are
erasable. Thus, it is possible to program the switch behavior of
the materials prepared from the SPBM, to achieve some
tailored stimuli-responsive behavior.
À2
tens of hours still canꢀt completely achieve the reversed
conversion, due to the robust dual-restrained effect induced
by the intermolecular anchoring effect and the stress-strain of
completely frozen polymer chains at a glassy state.
Based on these above distinct interconversion behavior of
the SPBM between the SP and MC form, programmable
triple-switchable effects on color, photoluminescence, and Tg
can be endowed to the materials prepared from this SPBM. In
the mode of thermal printing, a “butterfly” pattern can easily
thermally print on the film obtained via 520 nm light photo- Conclusion
polymerization (Figure 6A). This “butterfly” pattern is blue-
violet color (Figure 6B-i) and emit red fluorescence under
UV light (Figure 6B-ii), which can sustainably maintain in
dark and rapidly erase by bright visible light irradiation. In
the mode of photonic printing, the region of a “smiling face”
pattern was photopolymerized via 365 nm UV light with
a photomask at first, and then the whole region of the film was
In this work, we successfully prepared a cross-linkable
spiropyran-based molecule with triple-switchable effects on
color, photoluminescence, and mesomorphism, whose inter-
conversion behavior between the spiro and merocyanine form
in the different conditions have marked differences. Based on
the traits, it is possible to endow the materials prepared from
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Acknowledgements
This work was supported by the National Key R&D Program
of China (2020YFE0100200 and 2018YFB0703704) and Na-
tional Natural Science Foundation of China (Grant Nos.
5
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Manuscript received: May 26, 2021
Accepted manuscript online: June 23, 2021
15848.
Version of record online: && &&
,
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www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 9
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Research Articles
Responsive Materials
A cross-linkable spiropyran-based mole-
cule (SPBM) containing mesogenic units
with switchable color, photo-
W. Hu, C. Sun, Y. Ren, S. Qin, Y. Shao,
L. Zhang, Y. Wu,* Q. Wang,* H. Yang,*
luminescence and mesomorphism is
synthesized. Therefore, programmable
chromism and photoluminescence can
be endowed to the functional materials
prepared from the SPBMs, which own
a tunable glass transition temperature
D. Yang
&&&& — &&&&
Programmable Chromism and
Photoluminescence of Spiropyran-Based
Liquid Crystalline Polymer with Tunable
Glass Transition Temperature
(T ) by converting the configuration of the
g
crosslinked SPBMs to change the
molecular order degree.
Angew. Chem. Int. Ed. 2021, 60, 2 – 9
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!