Broadband Reflection in Polymer-Stabilized Cholesteric Liquid Crystals via Thiol–Acrylate Chemistry

Article abstract of DOI:10.1002/anie.201902681

Thiols are prone to react with a multitude of various functional groups in high yields, which has been widely used for surface- and particle-patterning, bioorganic synthesis, polymer modification, imprint nanolithography, the fabrication of optical components, hydrogel synthesis, and the curing of hard protective coatings. In this work, a chiral thiol with a high helical twisting power was synthesized through a novel synthetic route with high selectivity, yield, and cost-effectiveness. It was then used to fabricate a liquid-crystal composite film with ultra-wide broadband reflection via thiol click chemistry. Cholesteric liquid-crystal materials with broadband reflection have many potential applications for broadband polarizers, polarizer-free displays, organic optical data storage media, smart switchable reflective windows, and continuous waveband laser protection.

Full text of DOI:10.1002/anie.201902681

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Broadband Reflection in Polymer Stabilized Cholesteric Liquid
Crystals via Thiol–Acrylate Chemistry
Authors: Wei Hu, Mei Chen, Qian Wang, Lanying Zhang, Xiaotao
Yuan, Feiwu Chen, and Huai Yang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902681
Angew. Chem. 10.1002/ange.201902681
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10.1002/anie.201902681
Angewandte Chemie International Edition
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Broadband Reflection in Polymer Stabilized Cholesteric Liquid
Crystals via Thiol–Acrylate Chemistry
Wei Hu,a Mei Chen,b Qian Wang,b Lanying Zhang,b,c Xiaotao Yuan,a Feiwu Chena and Huai Yangb,c*
Abstract: Thiols are prone to react to high yields with a multitude of
various functional groups, which have been widely used for surface
and particle patterning, bioorganic synthesis, polymer modification,
imprint nanolithography, fabrication of optical components, hydrogel
synthesis, curing of hard protective coatings. In this work, a chiral thiol
with a high helical twisting power was synthesized through an
ingenious synthetic route with high selectivity, yield and cost-
effectiveness, which was used to fabricate a liquid crystal composite
film with ultra-wide broadband reflection via thiol-click chemistry.
Cholesteric liquid crystal materials with broadband reflection have
many potential applications for broadband polarizers, polarizer-free
displays, organic optical data storage media, smart switchable
reflective windows and continuous waveband laser protection.
is well known, thiol-click reactions have been extensively used to
fabricate functional polymer composites.[8] The thiol monomers
are not able to homopolymerize, therefore, the employment of a
thiyl chiral provider can facilitate the fabrication of pitch gradient
distribution.
Cholesteric liquid crystal (CLC) presents a unique kind of liquid
crystalline phase where the rodlike molecules self-assemble into
a periodic helical structure, exhibiting special selective Bragg
reflection to light.[1] The reflection wavelength is related to the
helical pitch p of the CLCs, whose magnitude can be adjusted
efficiently from tens of nanometers to micrometers by employing
appropriate dosage and helical twisting power (HTP, β) of the
chiral dopant.[2] It is known that p=1/(β.C) and λ=n.p, where
n=(no+ne)/2. Hence, the reflection bandwidth is △λ=△n.p, here
△n=ne–no is the birefringence of the LC.[3]
In prior studies, many effective methods had been presented to
control the reflection band of cholesteric LC.[2c, 4] For example,
Broer et al. pioneered a way to obtain a CLC film with broad
bandwidth covering the entire visible spectrum by obtaining a
pitch gradient.[5] Then, Mitov et al. fabricated a pitch gradient in a
cholesteric structure using glass-forming cholesteric oligomers[6].
Moreover, Yang et al. proposed several effective experimental
methods to achieve broadband reflection[7]
Therefore, fabricating CLC materials with pitch gradient is an
effective approach to obtain the broadband reflection property.[2c]
Usually, the chiral acrylate monomer was used to obtain the CLCs
with broadband reflection.[5] However, the in-situ radical
polymerization rate of acrylate is too high to guide further. [3b] As
Scheme 1. Reagents and conditions: a) p-TsCl, pyridine, DCM; 0 oC, 1h (90%).
b) KOH, EtOH; 80 oC, 20 h (88%) c) DCC, DMAP, DCM; 0 oC, 24 h (85%). d) p-
TsOH, H2O, toluene; reflux, 8h (quantitative). e) 3-Mercaptopropionic acid，p-
TsOH, toluene; Dean-Stark distillation, 16h (80%)
Isosorbide derivatives are a kind of fine chiral dopants with high
HTP and miscibility with LCs , whose acrylic derivatives usually
were used to fabricate CLC polymer composite.[9] Comparing to
the acrylic derivatives, the synthesis of terminal thiyl isosorbide
derivatives with similar structure is used to be more
complicated.[10] Here, we present an ingenious synthetic route to
synthesize an isosorbide derivative with double terminal thiyl
functional group, and the synthesis route is outlined in Scheme
1. Since 4-halogeno-butan-1-ol can’t exist stably in an alkaline
condition (yielding tetrahydrofuran)[11], it is hard to obtain 4-(4-
hydroxybutoxy)benzoic acid group with a high yield by general
etherification condition. Here, 4-(vinyloxy)butan-1-ol was used as
the etherification agent, compound 2 successfully was obtained
and hydroxyl group protection process needed in the next
esterification step was eliminated. Moreover, compound 4 can be
easily quantitatively obtained by acidifying compound 3, then the
target material was achieved via an esterification reaction by
using Dean-Stark distillation trap. Therefore, this synthesis route
has a high selectivity, yield and cost-effectiveness.
[a]
Wei Hu, Xiaotao Yuan and Feiwu Chen
Department of Chemistry and Chemical Engineering, School of
Chemistry and Biological Engineering,
University of Science and Technology Beijing,
Beijing 100083, People’s Republic of China.
Mei Chen, Qian Wang, Lanying Zhang and Huai Yang
Department of Materials Science and Engineering, College of
Engineering,
[b]
Peking University
Beijing 100871, People's Republic of China
E-mail: yanghuai@pku.edu.cn
[c]
Lanying Zhang and Huai Yang
Key Laboratory of Polymer Chemistry and Physics
of Ministry of Education
Peking University
Beijing 100871, People's Republic of China
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201902681
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Then, the photopolymerization of this thiol (RIS) and a liquid
crystalline acrylate monomer (C6M) in a host NLCs was used to
fabricate a polymer stabilized cholesteric liquid crystals (PSCLCs)
film with broadband reflection property.
exposed to a UV light, a UV-light-intensity gradient occurred in the
transverse direction resulting from the effect of UV absorbing dye.
The thiol–acrylate reaction was initiated by the UV light, and the
rate of the polymerization at the upper side of the film was larger
than the lower one due to a higher intensity of the UV light.[5] C6M
migrated constantly upwards due to its depletion near the upper
side, meanwhile RIS also migrated together with C6M in the
process of thiol-acrylate reaction.[14] Therefore, a concentration
gradient of RIS generates, and a pitch gradient of the CLC film
was obtained. The chemical compositions of the materials used
are shown in the Figure 1c.
Since thiols are unable to homopolymerize, while acrylate
monomers have two possible depletion way in the
photopolymerization, thus the molar ratio of functional groups is a
crucial factor to the conversion of thiols.[12b, 13a] Samples A1-A4
comprising different molar ratios of -SH and C=C were listed in
Table 1, and they were filled into a 50 μm-thick cell with planar-
alignment layers. All samples were irradiated by a 365 nm UV light
for 8 h at an intensity of 0.2 mW/cm2, then their conversions of
monomers were investigated by Fourier transform infrared
spectroscopy (FTIR) and gel fraction measurement. As shown in
Figure 2a, the characteristic infrared absorbance bands used to
monitor the disappearance of the functional groups during the
polymerization were as follows: acrylate (C=C) at 1407cm–1, and
13a]
thiol (S-H) at 2570 cm–1.[12b,
Unfortunately, there is an
overlapped absorbance bands near 2570 cm–1 in the spectrum of
C6M, therefore, the conversion of RIS cannot be clearly
determined only by FTIR. The acrylate (C=C) absorption peaks of
C6M all disappear in Samples A1-A4 (Figure 2b), which indicates
that the conversion of C6M reaches about 100.0% in all the
samples. That is, the weight loss in the gel fraction measurement
was mainly attributed to the unreacted thiols. The gel fractions of
Samples A1-A4 are 80.8 wt%, 90.1 wt%, 97.3 wt%, 97.6 wt%,
respectively, so the calculated conversion values of the thiol are
57.8 %, 74.3 %, 91.9 %, 91.8 %. The conversions of thiol raise
with the increase of C6M from Samples A1 to A3 and then keep
a constant value about 92.0% in Sample A4, indicating that the
conversion of thiol reach the maximum value when the molar ratio
of -SH and C=C was about 1.0:2.0. Furthermore, it means that the
depletion of acrylate in thiol-ene and homopolymerized reaction
is approximately equal.
Because RIS is the only chiral agent in this PSCLCs film,
theoretically, the pitch gradient distribution of the film is
determined by the conversion of RIS.[15] Therefore, there are
broader reflection bandwidth in Samples A3 and A4 comparing to
A1 and A2 (Figure 2c). Although more C6M is beneficial to the
conversion of RIS, but the planar textures will be disturbed when
excess monomers participate in the photopolymerization and the
migration. Therefore, the reflectivity of Sample A4 is decreased
distinctly comparing to Samples A1-A3, but the reflection
bandwidth is approximate to Sample A3. To sum up, 1.0:2.0 is the
optimal molar ratio of -SH and C=C for preparing the PSCLCs film
with broadband reflection. Then, Sample A3 was filled into two
cells, one of which has alignment layers and the other without.
Moreover, another planar oriented cell was filled by Sample A3
abandoning the dye. Three samples were irradiated for 2 h at 0.2
mW/cm2, and their transmittance spectra were compared to verify
the effect of the alignment layers and the dye, respectively. Figure
Figure 1. a) Radical initiation, propagation, chain-transfer, and
homopolymerization procedure in thiol−acrylate chemistry. b)
Schematic mechanism of the fabrication of PSCLCs film with
broadband reflection. c) Materials used in this system.
Thiol-acrylate photopolymerization is a binary reaction process,
in which the acrylic radical participates in the photopolymerization
process of a mixture of acrylate and thiol monomers: addition to
another acrylate functional group or hydrogen abstraction/chain
transfer to thiol.[12] This interlaced reaction modes of
photopolymerization lead to complex polymer network structures,
which comprise intertwined thiol-ene and homopolymerized
acrylate segments.[13] The factors governing the kinetics of
thiol−acrylate polymerization are the propagation rate (kPS), the
chain-transfer rate (kCT) and the homopolymerization rate (kHP),
the schematic of this reaction is shown in Figure 1a. The rate of
termination reactive that thiyl radicals (RS·) participated can be
neglected comparing with the one of propagation, and the rate of
a thiyl radical reaction with an acrylic double bond (kCT) is
approximate to the one of hydrogen abstraction by an acrylic
radical (kPS).[12b, 13a]
The schematic for fabrication of PSCLCs film with broadband
reflection is proposed in Figure 1b. A mixture comprising
RIS/C6M/Dye/SLC1717/DMPA were homogenized and filled into
a LC cell, forming a planar oriented CLC film. Then the cell was
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2d indicates that the alignment layer is indifferent in the fabrication
process of this film, because there is a flow alignment effect in the
sheared cholesteric liquid crystals during the process of flow into
the cell.[16] The fine planar textures of Sample A3 in a unalignment
cell before and after UV irradiation are obtained by POM as shown
in Figure S3. Moreover, a narrow reflection bandwidth was
obtained without the dye (Figure 2d), it reveals that the dye plays
a crucial role in obtaining a pitch gradient.[5]
for different intensities and time are shown in Figures 3a-d, and
the intensities are 5 mW/cm2, 1 mW/cm2, 0.2 mW/cm2 and 0.05
mW/cm2, respectively. The polymerization and migration occurred
at the same time with the irradiation beginning, hence the
reflection bandwidth became broader with the irradiation time.
Comparing the reflection bandwidth in Figures 3a-d, it indicates
that a wider bandwidth will be obtained by using a lower intensity
of UV light. If the intensity is too high, the UV intensity gradient
induced by dye is not distinct and the Rp is so large that the
monomers are immobilize by the polymer networks before
migration. At 0.05 mW/cm2, the widest bandwidth exceeds 2000
Table 1. Different Compositions of Samples
Sample
NO.
RIS/C6M/Dye/SLC-1717/DMPA
wt %
-SH: C=C
nm (Figure 3d).
Molar Ratio
The selective reflection band of CLCs can be altered by
adjusting the pitch via the concentrate of the chiral agent.
Samples comprising 3.0 wt%, 3.5 wt%, 4.0 wt%, 4.5 wt%, 5.0 wt%
and 6.0 wt% RIS (Samples A5-A10) were used to fabricate the
PSCLCs films, respectively. As shown in Figure 3e, the selective
reflection band generate a blue-shift with the increase of RIS,
hence the broadband reflection can efficiently adjust to any
optional wavelength to satisfy the requirement of application.
Meanwhile, Samples A5, A9 and A10 were filled into 20 μm-thick
cells to fabricate the PSCLCs films(Figure 3f), comparing with
those in 50 μm-thick cells, the bandwidths are narrower because
of the restricted space for gradient distribution of pitch.
A1
A2
5.0/6.0/1.0/88.0/0.5
5.0/8.0/1.0/86.0/0.5
5.0/10.0/1.0/84.0/0.5
5.0/12.0/1.0/82.0/0.5
3.0/6.0/1.0/80.0/0.5
3.5/7.0/1.0/78.5/0.5
4.0/8.0/1.0/77.0/0.5
4.5/9.0/1.0/75.5/0.5
5.0/10.0/1.0/74.0/0.5
6.0/12.0/1.0/77.0/0.5
5.0[a]/10.0/1.0/74.0/0.5
~1.0:1.2
~1.0:1.6
~1.0:2.0
~1.0:2.4
~1.0:2.0
~1.0:2.0
~1.0:2.0
~1.0:2.0
~1.0:2.0
~1.0:2.0
-
A3
A4
A5
A6
A7
A8
A9
A10
A11
[a] RIS is replaced by RIA.
Figure 2. Characterization of the reaction kinetics and transmittance spectra of
the sample comprised of different compositions. a) FTIR spectra of C6M, RIS
and LC1717. b) FTIR spectra of Samples A1-A4 before and after UV irradiation
for 8 h at 0.2 mW/cm2. c) Transmission spectra of Sample A1-A4 after UV
irradiation. d) The influence of dye and alignment to transmission spectra.
Obviously, the polymerization rate (Rp) and conversion rate of
the photopolymerization are correlative to the intensity of UV light
and irradiate time, and the performance of the PSCLCs film is also
impacted by these factors.[13a] Moreover, the migration rate (Rm)
of the monomers in the photopolymerization is another crucial
factor to the property of the materials. The monomers
polymerizing in-situ or upwards are determined by both the Rp and
the Rm: when the polymerization rate of the upper position Rpu is
equal to lower one Rpl, the monomers will not migrate upwards,
indicating that Rm tends to be zero; thus, only if the Rpu >> Rpl, a
larger Rm can be obtained. Since the Rp is controlled by the UV
intensity, a UV intensity gradient will induce a Rp gradient. The
transmission spectra of Sample A3 irradiated with 365 nm UV light
Figure 3. Transmission spectra of the samples. a) Sample A3 before irradiation
and after irradiation for different time at 5 mW/cm2; b) 1 mW/cm2; c) 0.2 mW/cm2
and d) 0.05 mW/cm2. e) Samples A5-A10, 0.2 mW/cm2 6 h. f) Samples A5, A9
and A10 in 20 μm-thick cells, 0.2 mW/cm2 6 h. g) Sample A11 before irradiation
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10.1002/anie.201902681
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and after irradiation at 0.05 mW/cm2 for different time. h) Comparison of the
reflection bandwidth of Samples A3 and A11 after irradiation at 0.05 mW/cm2
until to the constant state.
thiol-acrylate chemistry to prepare a PSCLCs film with a reflection
bandwidth over 2000 nm which is prominently wider than the pure
acrylate system. The influence factors in fabrication procedure
were systematically investigated and the mechanism was
explicitly verified by SEM and EDS. Thus, this work indicates a
great potential of thiol−click chemistry in designing and preparing
functional materials with tailored properties for practical
applications.
In prior studies, chiral acrylate monomers usually was used to
fabricate the CLC film with broadband reflection property.[5] To
make a comparison, a chiral diacrylate monomer RIA with the
similar structure to the RIS was synthesized. The HTP values of
o
RIA is 34.1 μm–1 at 25 C in the host LCs SLC1717, which is
approximate to RIS (33.6 μm–1). Sample A11 composing 5.0 wt%
RIA, 10.0 wt% C6M, 1.0 wt% Dye, 0.5 wt% DMPA and 84.0 wt%
SLC1717 was filled into a 50 μm-thick cell and irradiated at 0.05
mW/cm2. After 30 min, the reflection bandwidth stops broadening
with the irradiation, which indicates that the migration of
monomers has finished (Figure 3g). The widest reflection
bandwidth from RIA is about 1100 nm, which is approximately half
of the one from RIS at the same fabrication condition (Figure 3h).
This result can attribute to the high Rp in this system, the in-situ
homopolymerization of RIA result in that the chiral monomers are
immobilize by the polymer networks before migration.
To verify the mechanism, the microstructures of the films from
Samples A3 and A5 was observed by a field emission scanning
electron microscopy (SEM).[17] From the SEM images of the
fractured surface of the films, the distribution of the self-organized
helical pitch gradient from top to bottom can be evidently
observed (Figure 4a-i, p1<p2<p3 and 4b-i, p4<p5<p6). The
energy-dispersive X-ray spectrometry (EDS) mappings of O and
S elements were used to characterize the distribution of the
monomers in the PSCLCs. It can be seen that both O and S
exhibit obvious content gradient from top to bottom (Figures 4a-ii,
iii and 4b-ii, iii). The content of O and S in RIS are 27.16% and
9.07% according to the chemical formula C34H42O12S2, and the
contents of O in C6M is 23.78% according to the chemical formula
C39H44O10. There are high content of O in both monomers, but RIS
is the only source of S. Therefore, the gradient distribution S
indicates that the RIS had extensive migration in the
polymerization, which explicitly confirm the mechanism proposed
above.
Experimental Section
A room temperature commercial nematic LC, SLC-1717 (SLC Co. Ltd. no = 1.519, ne
=
1.720, △n= ne-no = 0.201, TCr-N<－40 oC, TN-I=91.8 oC), was used as the host LC. Isosorbide
bis(4-(4-((3-mercaptopropanoyl)oxy)butoxy)benzoate) (RIS) was synthesized in our lab,
whose HTP value is about 33.6 μm–1 at 25 oC in a host LCs SLC1717. The acrylate was a
conventional photopolymerizable LC monomer, 1,4-di-[4-(6-acryloyloxy) hexyloxy
benzoyloxy]-2-methyl
benzene
(C6M, HCCH Co.
Ltd.).
2,2-dimethopxy-1,2-
diphenylethanone (DMPA, TCI Co. Ltd.) and 6-[4-[2-[4-(Hexylsulfonyl) phenyl] ethenyl]
phenoxy]-1-hexanol (Dye, Merck Co. Ltd.) were used as the UV-initiator and UV-absorber,
respectively. Isosorbide bis(4-(4-(acryloyloxy)butoxy)benzoate) (RIA), the HTP value of RIA
is about 34.1 μm–1 at 25 oC in the host LCs. The syntheses of RIS and RIA are described in
the Supporting Information.
The optical textures of the films were observed using a polarizing optical microscope (POM,
Carl Zeiss Axio Vision SE64). HTP value were measured with the Cano-Wedge technique.
The kinetics study of model thiol-acrylate was investigated by Fourier transform infrared
spectroscopy (FTIR, Bruker, Vector-22). The gel fractions were obtained by measuring the
residual weight of the sample in the cell, which were immersed in adequate dichloromethane
for about 48 h after photopolymerization and then dried to constant weight. The selective
reflection spectra of the films were obtained by a UV/Vis/NIR spectrophotometer (Perkin
Elmer, Lambda 950), and the transmittance of a blank cell was normalized as 100.0%. The
microstructures of the PSCLCs films after samples quenched to rupture by liquid nitrogen
were observed by a field emission scanning electron microscopy (SEM, Hitachi S-4800)
equipped with an Inage J as a software tool and an energy-dispersive X-ray spectrometer
(EDS, Bruker Quantax).
Acknowledgements
This work was supported by the National Key R&D Program of
China (2018YFB0703704), the Joint Fund of the Ministry of
Education for Equipment Pre-research (Grant no. 6141A020222),
the National Natural Science Foundation of China (NSFC)
International Cooperation and Exchanges Projects (Grant no.
51720105002) and NSFC (Grant no. 51561135014).
Corresponding Author
Huai Yang, yanghuai@pku.edu.cn
Author Contributions
Figure 4. SEM images and EDS mappings of the fractured surface in the
PSCLCs films. a) SEM image (i), EDS mapping of O element (ii and S element
(iii) of Sample A3. b) SEM image (i), EDS mapping of O element (ii) and S
element (iii) of Sample A5.
The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the
manuscript.
In conclusion, we present an ingenious synthetic route with
high selectivity, yield and cost-effectiveness for fabricating a rigid
chiral thiol with terminal thiyl functional group, and then
successfully demonstrated a simple and effective method via
Notes
The authors declare no competing financial interest.
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10.1002/anie.201902681
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Keywords: thiol-acrylate chemistry • chiral thiol • liquid crystals •
broadband reflection • photopolymerization
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10.1002/anie.201902681
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Wei Hu,a Mei Chen,b Qian Wang,b
Lanying Zhang,b Xiaotao Yuan,a Feiwu
Chena and Huai Yangb*
Page No. – Page No.
Broadband Reflection in Polymer
Stabilized Cholesteric Liquid Crystals
via Thiol–Acrylate Chemistry
A polymer stabilized cholesteric liquid crystals film with broadband reflection property
was obtained via thiol–acrylate chemistry, whose pitch gradient distribution was
clearly presented by scanning electron microscope and the migration of the
monomers in photopolymerization was explicitly verified by energy-dispersive X-ray
spectrometry.
[a]
[b]
Wei Hu, Feiwu Chen
Department of Chemistry and Chemical Engineering, Scho
Chemistry and Biological Engineering,
University of Science and Technology Beijing,
Beijing 100083, People’s Republic of China.
Mei Chen, Qian Wang, Lanying Zhang and Huai Yang
Department of Materials Science and Engineering, College
This article is protected by copyright. All rights reserved.