Application of double click to prepare near-infrared absorbing dye for photo-thermal tuning of cholesteric liquid crystal

Article abstract of DOI:10.1016/j.reactfunctpolym.2020.104549

Cholesteric liquid crystal (CLC) exhibits selective reflection, making it a great application in the field of light control. Here, a near-infrared light (NIR) was used to drive the self-organizing spiral superstructure of the cholesteric liquid crystal to

Full text of DOI:10.1016/j.reactfunctpolym.2020.104549

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Application of double click to prepare near-infrared absorbing dye
for photo-thermal tuning of cholesteric liquid crystal
Zhitao Li, Dong Wang, Hong Gao, Hui Cao, Yuzhen Zhao,
Zongcheng Miao, Zhou Yang, Wanli He
PII:
S1381-5148(19)31285-4
DOI:
https://doi.org/10.1016/j.reactfunctpolym.2020.104549
REACT 104549
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2 December 2019
4 March 2020
6 March 2020
Please cite this article as: Z. Li, D. Wang, H. Gao, et al., Application of double click
to prepare near-infrared absorbing dye for photo-thermal tuning of cholesteric liquid
crystal, Reactive and Functional Polymers (2019), https://doi.org/10.1016/
j.reactfunctpolym.2020.104549
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Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
Application of double click to prepare near-infrared absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
a
a,
b,
a,
c
Zhitao Li, Dong Wang, * Hong Gao, * Hui Cao, * Yuzhen Zhao, Zongcheng
c
a
a
Miao, Zhou Yang and Wanli He
Dong Wang, School of Materials Science and Engineering, University of Science
and Technology Beijing, Address: University of Science and Technology Beijing
Corrosion Building 608, Beijing 100083, PR China, E-mail:
wangdong@ustb.edu.cn, phone number: 008618611141923
Hui Cao, School of Materials Science and Engineering, University of Science and
Technology Beijing, Address: University of Science and Technology Beijing
Corrosion Building 608, Beijing 100083, PR China, E-mail:
caohui@mater.ustb.edu.cn, phone number: 00861062333759
Hong Gao, Division of Material Engineering, China Academy of Space Technology,
Beijing 100094, PR China, E-mail: gaohong_cast@sina.com, phone number:
0
08618701566189
1

Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
Application of double click to prepare near-infrared absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
a
a,
b,
a,
c
Zhitao Li, Dong Wang, * Hong Gao, * Hui Cao, * Yuzhen Zhao, Zongcheng
c
a
a
Miao, Zhou Yang and Wanli He
a
School of Materials Science and Engineering, University of Science and Technology
Beijing, Beijing, 100083, PR China
b
Division of Material Engineering, China Academy of Space Technology, Beijing,
1
00094, PR China
c
Key Laboratory of Organic Polymer Photoelectric Materials, School of Sciences,
Xijing University, Xi’an, 710123, PR China.
Abstract
Cholesteric liquid crystal (CLC) exhibits selective reflection, making it a great
application in the field of light control. Here, a near-infrared light (NIR) was used to
drive the self-organizing spiral superstructure of the cholesteric liquid crystal to adjust
the reflection notch. The polymer dye M2 with near-infrared absorption properties was
synthesized by thiol-ene click polymerization and [2 + 2] click reaction; and has good
solubility in the CLC system. M2 absorbs near-infrared light to increase the temperature
of the system and the temperature-sensitive chiral dopant CD can realize the
photothermal tuning of CLC system by the spiral twisting force of CLC with the change
of temperature. The use of near-infrared light is beneficial compared to ultraviolet (UV)
2

Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
and visible light because of its invisibility, high penetration and ease of remote control.
The entire CLC system has good near-infrared absorption, fast light response and a
wide adjustable range.
Keywords: Cholesteric liquid crystal; Near-infrared light; Click reaction;
Photothermal tuning.
3

Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
1
. Introduction
Responsive materials have attracted much attention for their great application value.
In general, the reflected light of responsive materials is based on periodic structures
rather than dyes that is the basic strategy for most organisms to transmit optical signals
1
in nature. When a creature is exposed to external stimuli, it will voluntarily produce a
change in color. Scientists have designed many responsive materials through similar
principles. They can not only obtain a variety of color changes, but also can be adjusted
2
-4
5
6, 7
8
in a variety of ways, such as light,
heat, electricity,
stress and so on.
Cholesterol liquid crystals (CLCs) - also known as chiral nematic liquid crystals (N*-
9
, 10
LC) have a spontaneous self-assembling periodic helical superstructure.
The spiral
superstructure of CLC is characterized by two structural parameters: the handedness and
the pitch. The direction in which the director rotates about the helical axis defines its
direction of rotation, while the pitch length (p) corresponds to the distance over which
1
1
the director rotates a full turn. It is well known that the orientation of CLC molecules
changes in a spiral manner, so that light is selectively reflected according to Bragg's
1
2
law. The selective reflection is determined by the pitch of the helix in the CLC and
can be adjusted due to the dynamic control of the self-organizing helical superstructure
by external stimuli. CLC's selective reflection has many applications, such as tunable
1
3-15
16, 17
lasers,
optical switches
and information storage. However, in a variety of easily
available stimuli, light has many advantages such as remote, local, spatial and temporal
controllability, rapid response and so on.
Currently, the tuning methods of photothermal and electrothermal effects have
attracted more and more attention. Wang et al designed a self-organized helical
superstructure impregnated with unique mesogen- modified gold nanorods. The inertial
4

Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
inversion was shown when the NIR was irradiated, resulting from the significant
1
8
photothermal effect of the embedded gold nanorods. Wu et al proposed a new method
based on the electrothermal effect. By applying an AC voltage, the reflection color can
1
9
be adjusted in a wide wavelength range to cover the entire visible spectrum. Hsiao et
al prepared an organic membrane with a self-organized spiral superstructure. The
organic film is a dual-frequency thermosensitive cholesterol (DFTC) material, which is
composed of a liquid crystal body doped with a binary chiral dopant mixture including a
thermal chiral switch. The red, green and blue reflections in the DFTC film can be
obtained by causing the dielectric to heat by changing the frequency of the applied
20
voltage of 45 Vrms. Based on the above-mentioned literature investigation, we doped
polymer near-infrared dye into the CLC system for the first time and used NIR light to
regulate the spiral structure of CLC. The most common means of photoresponsive CLC
preparation is to add a small amount of photochromic chiral molecular switches, such as
2
1, 22
azobenzenes, dialkylethylenes, etc. to the nematic LC host.
However, such systems
generally require ultraviolet (UV) or visible light stimulation. High-energy ultraviolet
rays are harmful to the human body and have poor penetrability that easily leads to
material aging and environmental pollution. Compared with ultraviolet/visible light,
near-infrared light has the advantages of invisibility and strong penetrating power, and
can remotely control the spiral upper layer structure. It has great application value in
special fields such as life science, aerospace and other fields.
Therefore, a polysiloxane side chain liquid crystal polymer dye M2 was synthesized
with near-infrared absorption properties by thiol-ene click polymerization and [2 + 2]
2
3-26
click chemistry.
Two side chains with different functions are branched onto the
flexible main chain PMMS (Poly[3mercaptopropylmethyl-siloxane]), one is the liquid
5

Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
crystal monomer AC-6CN, and the other is an aniline derivative having a clickable
triple bond. The presence of AC-6CN allows M2 to have better solubility in the CLC
system, while the aniline derivative after F4-TCNQ click has strong absorption
characteristics in the near-infrared region. M2 was added to the CLC system and
equipped with a chiral dopant CD with temperature-sensitive properties. Under NIR
light stimulation, the entire CLC has good absorption, fast responsiveness and a wide
tunable wavelength range.
2
2
2
. Experimental
.1 Materials and instrumentations
.1.1 Materials
Poly[3-mercaptopropylmethyl-siloxane] (PMMS, SMS-992, M.W. 4000-7000, 95 cst)
was purchased from Gelest Inc. 2,2-Dimethoxy-2-phenylacetophenone (DMPA) and
copper (I) iodide were purchased from Aladdin Inc. 2,3,5,6-tetrauoro-7,7,8,8-
tetracyanoqui-nodimethane (F4-TCNQ), 4-iodophenol, 4-iodoaniline, 1-bromobutane
and 3-bromoprop-1-ene were purchased from Energy Chemical. Trans-Dichlorobis
(triphenyl-phosphine) Palladium (II) and Trimethylsilylacetylene (TMSA) were
purchased form Sinopharm. Potassium iodide, Anhydrous acetonitrile, Anhydrous
methanol (MeOH) and Potassium carbonate were purchased from Beijing Dominant
Technology Co. These chemicals were of commercially analytical grade.
Dichloromethane (DCM), triethylamine (TEA) and N, N-Dimethylformamide (DMF)
were distilled on CaH2 under nitrogen. Tetrahydrofuran (THF) was distilled on sodium
benzophenone ketyl undernitrogen. Other chemical reagents were used without further
purification.
6

Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
LC, (SLC-1717, no = 1.519, ne = 1.720, n = ne - no = 0.201, TN-I = 92°C) was
purchased from Shijiazhuang Yongsheng Huatsing Liquid Crystal Co. Ltd. 6-((4'-
cyano-[1,1'-biphenyl]-4-yl)oxy)hexyl acrylate (AC-6CN) as liquid crystal side chain for
M1 was purchased from Shijiazhuang Yongsheng Huatsing Liquid Crystal Co. Ltd.
(+)[1,1'-binaphthalene]-2,2'-diyl bis(4'-(2-methylbutyl)-[1,1'-biphenyl]-4-carboxylate)
2
7
as a chiral dopant, it was synthesized in our previous work. The molecular formulas of
chiral dopant and side chain liquid crystal monomers are shown in Figure 1.
Figure 1. Chemical structure of chiral dopant CD and AC-6CN.
2
.1.2 Instrumentations
1
All H-NMR spectra were obtained using BRAKER AVANCE III HD NMR
spectrometer (500 MHz) at 25°C using deuterated chloroform (CDCl3) with
tetramethylsilane (TMS) as the internal standard. The FT-IR spectra were collected
using a Perkin Elmer LR-64912C Fourier transform infrared spectrometer.
Thermogravimetric analysis (TGA) was performed on a TAQ600 at a heating rate of
-
1
1
0°C · min under nitrogen. Gel permeation chromatography (GPC) was performed on
an HP 1100 high pressure liquid chromatography (HPLC), equipped with an HP 1047A
refractive index detector and a Plgel MIXED-C 300-7.5 mm column (packed with 5 μm
7

Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
particles). The column packing allowed the separation of polymers over awidemolecular
weights (200-3000000g/mol). THF was used as the eluent at a low flow rate of 1
-
1
mL·min at 35°C Polystyrene standards were used as the references. All MALDI-TOF-
MS spectra were recorded on a Shimadzu AXIMA-CFR mass spectrometer. The
operation was performed at an accelerating potential of 20 kV in a linear positive ion
mode with dithranol as a matrix. Elemental analyses were performed at the Institute of
Chemistry, Chinese Academy of Sciences, with a Flash EA 1112 instrument. The
reflection spectra and UV absorption were recorded in a quartz cuvette using a JASCO
V-570 spectrophotometer. Polarized optical microscopy (POM) was carried out to
observe the liquid crystal textures of the samples using a Carl Zeiss Axio Vision SE64
polarized optical microscope with a Linkam LTS420 hot stage. ST9450 handheld
thermal imager collects thermal imaging images. Power-adjustable BD-808 near-
infrared laser as light-tuned light source. Power-adjustable FUV-6BK UV curing device
for inducing UV polymerization.
2
.1.3 cell conditions
Two types of liquid crystal cells have been prepared. As shown in Figure 2.(One was
a glass liquid crystal cell. Figure 7 (b) (c) was given after measurement using a glass
liquid crystal cell. Another type used a flexible PET cell, and Figure. 7 (a) and Figure 8
used a PET cell) The preparation process of the LCD cell is as follows: Preparation
process of the liquid crystal cell: the glass containing the ITO layer was cut into a size
of 1 cm * 1.5 cm, cleaned and oriented. PET (polyethylene terephthalate) film (10 μm
thick) is used as cell spacers. The two pieces of glass were glued together through
spacers to prepare a liquid crystal cell. The LC, M2 and CD system was mixed
uniformly and filled into the cells by capillary action. Preparation of flexible PET liquid
8

Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
crystal cell: The transparent PET film (5 μm thick) was cut to a size of 1 cm * 1 cm and
cleaned. Add a small amount of the mixed LC system to the PET film and squeeze it
with another PET film to spread the liquid crystal uniformly.
Figure 2 Preparation process of glass liquid crystal cell and PET cell
2
2
.2 Synthetic procedures
.2.1 1-(allyloxy)-4-iodobenzene (1)
3-bromoprop-1-ene (1.21 g, 0.01 mol), 4-iodophenol (2.2 g, 0.01 mol) and anhydrous
potassium carbonate (1.38 g, 0.01 mol) were added in anhydrous acetonitrile (100 mL).
The mixture was stirred at 85°C for 3 h, filtration to remove inorganic salts, evaporation
and column chromatography (SiO2, Vpetroleum ether (PE)/Vdichloromethane(DCM) = 2:1) accorded
1
the desired product 1 as a colorless oily liquid with a yield of 96%. H-NMR (CDCl3,
5
5
00 MHz): δ = 5.57 (d, J = 9.0 Hz, 2H), 6.72 (d, J = 9.0 Hz, 2H), 6.00-6.10 (m, 1H),
.43 (d, J = 17.5, 1H), 5.32 (d, J = 10.5 Hz, 1H), 4.53 (d, J = 5.5 Hz, 2H) ppm. FT-IR
-
1
(
KBr, cm ): ν = 3083, 2865, 1586, 1487, 1286, 1245, 993, 812, 583, 501. MALDI-
-
1
-1
TOF-MS (dithranol): m/z: calcd for C9H9IO: 259.97 g · mol , found: 261.3 g · mol
9

Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
+
[
MH] ; elemental analysis calcd (%) for C9H9IO (259.97): C 41.56, H 3.49, O 6.16,
found: C 41.56, H 3.50, O 6.17.
2
.2.2 polymer (2)
Compound 1 (110 mg, 0.42 mmol), PMMS (275.2 mg, 2 mmol -SH), DMPA (20 mg
wt%) and 15 mL THF were added into a dry 20 mL glass vessel. The vessel was
2
radiated under a 50 W UV light (λmax = 365 nm) for 30 min with mild agitation. Then
AC-6CN (570 mg, 1.62 mmol) was added in the same glass vessel and continue to
irradiate for 30 min. The solution was poured into a large excess amount of methanol.
The precipitate was collected by filtration, washed with methanol and dried under
vacuum. The desired product 2 was a colorless viscous liquid, obtained via thiol-ene
1
click reaction in 85% yield. H-NMR (CDCl3, 500 MHz): δ = 7.78-7.45 (m, 2.22H),
7
2
1
7
2
.08-6.91 (m, 0.76H), 6.74-6.60 (m, 0.20H), 4.30-3.90 (m, 1.73H), 2.91-2.45 (m,
.04H), 2.09-1.99 (m, 0.22H), 1.92-1.41 (m, 4.76H), 0.76-0.53 (m, 1H), 0.22-0.04 (s,
-
1
.5H) ppm. FT-IR (KBr, cm ): ν = 3065, 2920, 1879, 1586, 1488, 1241, 1175, 1079,
92, 508, 434.
.2.3 N, N-dibutyl-4-iodoaniline (3)
To a degassed solution of 4-iodoaniline (4.38 g, 0.02 mol), 1-bromobutane (8.65 g, 0.05
mol) in N, N-Dimethylformamide (DMF) (100 mL), anhydrous potassium carbonate
(1.38 g, 0.01 mol) and potassium iodide (1.66 g, 0.01 mol) were added under Ar. The
mixture was refluxed under magnetic stirring at 120°C for 20 h. After removal of the
precipitated salt, the organic phase was washed three times with water, drying over
Na2SO4, the solution was filtered. Removal of the solvent in vacuo and purification on
column chromatography (SiO2, VPE/VDCM = 3:1) afforded 70% yield of the desired
1
product 3 as a Colorless oily liquid. H-NMR (CDCl3, 500 MHz): δ = 7.44 (d, J = 9.0
1
0

Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
Hz, 1H), 6.44 (d, J = 9.0 Hz, 1H), 3.25 (t, J = 7.5 Hz 2H), 1.53-1.61(m, 2H), 1.30-1.42
-
1
(
m, 2H), 0.98 (t, J = 7.5 Hz, 3H) ppm. FT-IR (KBr, cm ): ν = 3085, 2958, 2864, 1744,
1
588, 1469, 1392, 1287, 1184, 1100, 986, 906, 804, 697, 434. MALDI-TOF-MS
-
1
-1
+
(
dithranol): m/z: calcd for C H NI: 331.08 g · mol , found: 332.05 g · mol [MH] ;
14 22
elemental analysis calcd (%) for C H NI (331.08): C 50.76, H 6.69, N 4.23, found: C
1
4
22
5
0.77, H 6.70, N 4.23.
.2.4 N, N-dibutyl-4-((trimethylsilyl) ethynyl) aniline (4)
2
To a degassed solution of 3 (332 mg, 1.00 mmol) and trimethylsilylacetylene (TMSA)
147 mg, 1.50 mol) in triethylamine (TEA) (30 mL) and tetrahydrofuran (THF) (30
(
mL), bis (triphenylphosphine) palladium (II) dichloride (PdCl2(PPh3)2) (70.2 mg, 0.10
mmol) and cuprous iodide (CuI) (20 mg, 0.10 mmol) were added under Argon. The
mixture was refluxed under magnetic stirring at 40°C for 8 h. After removal of the
catalyst, solvent evaporation and purification on column chromatography (SiO2,
VPE/VDCM = 6:1) afforded 83% yield of the desired product 4 as a yellow oily liquid.
1
H-NMR (CDCl3, 500 MHz): δ = 7.32 (d, J = 9.0 Hz, 1H), 6.54 (d, J = 9.0 Hz, 1H), 3.29
(t, J = 8.0 Hz, 2H), 1.63-1.53 (m, 2H), 1.32-1.42 (m, 2H), 0.98 (t, J = 7.0 Hz, 3H),
-
1
0
1
.26(s, 4.5H)ppm. FT-IR (KBr, cm ): ν = 3098, 2953, 2897, 2148, 1874, 1609, 1515,
462, 1368, 1245, 869, 757, 535. MALDI-TOF-MS (dithranol): m/z: calcd for
-
1
-1
+
C19H31NSi: 301.22 g · mol , found: 302.3 g · mol [MH] ; elemental analysis calcd
%) for C19H31NSi (301.22): C 75.68, H 10.36, N 4.65, found: C 75.69, H 10.37, N
(
4
.65.
2
.2.5 N, N-dibutyl-4-ethynylaniline (5)
Compound 4 (1.508 g, 0.005 mol) and anhydrous potassium carbonate (692 mg, 0.005
mol) were dissolved in THF/MeOH (VTHF/VMeOH = 7:3, 100 mL). The mixture was
1
1

Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
refluxed under magnetic stirring at room temperature for 3 h; the inorganic salts were
then removed and evaporated. The organic layer was purified using a silica gel column
1
(
SiO2, DCM) obtaining a yellow-green oily product 5 with a yield of 98%. H-NMR
(
CDCl , 500 MHz): δ = 7.35 (d, J = 9.0 Hz, 1H), 6.56 (d, J = 9.0 Hz, 1H), 3.29 (t, J =
3
7
3
1
.5 Hz, 2H), 2.98 (s, 0.5H), 1.64-1.53 (m, 2H), 1.33-1.42 (m, 4H), 0.98 (t, J = 7.5 Hz,
-
1
H) ppm. FT-IR (KBr, cm ): ν = 3304, 3093, 2956, 2098, 1740, 1607, 1367, 1148,
007, 928, 814, 740, 529. MALDI-TOF-MS (dithranol): m/z: calcd for C16H23N: 229.18
-
1
-1
+
g · mol , found: 230.2 g · mol [MH] ; elemental analysis calcd (%) for C16H23N
229.18): C 83.79, H 10.11, N 6.10, found: C 83.79, H 10.12, N 6.09.
.2.6 polymers (M1)
Compound 2 (300 mg) and 5 (41.5 mg, 0.18 mmol) were dissolved in THF/TEA (VTHF
VTEA = 1:1, 20 mL). After the solution was degassed under nitrogen for 30 min, then
(
2
/
added PdCl2(PPh3)2 (120 mg, 0.17 mmol) and CuI (21 mg, 0.11 mmol). The reaction
mixture was stirred at 40°C for 8 h under Ar. The mixture was filtered to remove the
catalyst and mixed with a large excess amount of methanol. The precipitate was
collected by filtration. After washing with methanol and drying under vacuum, a yellow
-
1
gelatinous solid polymer M1 was obtained in a yield of 81%. FT-IR (KBr, cm ): ν =
2
918, 2848, 2223, 2194, 1732, 1642, 1601, 1491, 1382, 1246, 1175, 1072, 819, 533.
2
.2.7 polymers (M2)
M1 (150 mg) and F4-TCNQ (27.9 mg, 0.10 mmol) were dissolved in 1,2-dichloroethane
DCM) (4 mL) under Argon. The mixture was stirred for 3 h at 25°C and then the
(
solution was poured into a large excess amount of methanol. The precipitate was
collected by filtration, washed with MeOH and dried under vacuum. The desired
1
2

Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
-
1
product M2 was a green solid with a yield of 91%. FT-IR (KBr, cm ): ν = 2935, 2863,
196, 1733, 1604, 1493, 1397, 1250, 1177, 1073, 822, 534.
2
Figure 3. Synthesis of monomer M1 (a) K2CO3, Acetonitrile, 85°C, 3 h; (b) DMPA,
THF, 365 nm UV, 30 min; (c) K2CO3, KI, DMF, 120°C, Ar, 20 h; (d) TMSA,
PdCl2(PPh3)2, CuI, TEA, THF, 40°C, Ar, 8 h; (e) K2CO3, THF, MeOH 25°C, 3 h; (f)
CuI, PdCl2(PPh3)2, THF, TEA, 40°C, Ar, 8 h.
3
3
. Results and discussion
.1 Polymer Synthesis
The synthesis of polysiloxane polymer M2 mainly consists of two steps. In the first
step, the prerequisite polymer M1 was synthesized by thiol-ene addition reaction with
2
8
DMPA as initiator. In the second step, the partial side chain of M1 reacted with the
click reagent F4-TCNQ through [2 + 2] click chemical reaction to obtain M2. As shown
in Figure 3, 1-(allyloxy)-4iodobenzene and AC-6CN were grafted onto the main chain
PMMS (Poly[3-mercaptopropylmethyl-siloxane]) by UV polymerization. In order to
ensure the complete consumption of -SH, the method of step-by-step polymerization
was uesd: the dosage of the two reactants was determined according to the molar ratio
and then the 1-(allyloxy)-4-iodobenzene was UV-polymerized. After that the excess
1
3

Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
AC-6CN was added to the mixture and continued UV polymerization for 30 min. -SH
bond can be completely reacted. In addition, the monomer 5 was synthesized by
methylsilyl protection, and then reacted with polymer 2 by sequential Sonogashira
2
9
cross-coupling to form a side chain with an electron donating triple bond.
1
The chemical structure of the polymer was confirmed by FT-IR and H-NMR
1
spectroscopy. The H-NMR spectra of AC-6CN, 1, 5 and M1 are shown in Figure 4.
1
(
The H-NMR spectra of M2 shows in s1) On the basis of the integral ratio of the
methylene protons (0.72-0.60 ppm, Si−CH2−) of PMMS with the aromatic proton (6.92-
.04 ppm) of AC-6CN and the aromatic proton (6.52-6.60 ppm) of the aniline
7
derivative. The actual graft density can be calculated to give a result of around 100%;
the -SH bond was substantially eliminated. Terminal olefin proton peaks of AC-6CN
and 1-(allyloxy)-4-iodobenzene (1) at 5.82-6.51 ppm and 5.25-6.15 ppm were not
1
observed in the H-NMR spectrum of M1. The terminal acetylene proton peak at 2.98
ppm of the small molecule N, N-dibutyl-4-ethynylaniline (5) also did not appear in the
M1 which means that the monomer has been successfully grafted onto the PMMS
backbone and all unreacted monomers have been removed. All proton peaks have a
good correspondence with the nuclear magnetic spectrum. It is convinced that the thiol-
ene click chemistry is a simple and efficient method to graft mesogens onto
polysiloxane backbone.
1
4

Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
1
Figure 4. H NMR spectra of AC-6CN, 1, 5 and M1.
Figure 5 (a) shows the FT-IR spectrum of PMMS, polymers M1 and M2, there was
-
1
no absorption at 2557 cm for M2 assignable to the -SH stretching, indicating that the -
-
1
SH has been completely reacted. The absorption for M1 in the 2190-2230 cm was
assignable to the intense stretching vibration of -CN and RC≡CR. After the [2 + 2] click
-
1
reaction, the peak of this range was replaced by the more intense 2196 cm (assigned as
the -CN) stretching vibration peak. In addition, the partially amplified FT-IR spectra as
shown in Figure 5 (b). The appearance of the vibrational bands, which were derived
-
1
-1
-1
from ester carbonyl (C=O), N-C and Si-C at 1732 cm , 1491 cm and 1246 cm
providing evidence that the preparation of polymers via thiol-ene addition reaction went
to completion.
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Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
Figure 5. FT-IR spectra of M1, M2 and PMMS.
.2 UV and thermal performance characterization
3
A side chain liquid crystal polymer M2 having near-infrared absorption
characteristics was prepared by [2 + 2] click chemical reaction, as shown in Figure 6(a).
Click chemistry has excellent reactivity and no by-product characteristics. By clicking
chemical method, M2 has both near-infrared absorption and can be well compatible
with liquid crystals compared to other materials. M2 is added as an infrared absorber to
the liquid crystal system, so that the system has a good near-infrared absorption effect.
Figure 6 (b) shows the UV-vis absorption spectra of the polymers M1 and M2. M1
and M2 showed the same two peaks at 232 nm and 298 nm that are represented by the
absorption peak of the siloxane backbone and the absorption peak of AC-6CN. M1 has
a small absorption peak at 342 nm and this peak is expressed as the absorption peak of
the aniline derivative branch. The end absorption of the product M2 after the click
chemistry reaction was at 864 nm. It indicates that the click reaction causes the end
absorption (λend) of M1 to red shift due to the formation of the donor-acceptor
chromophore. This also proves that M2 has a certain absorption under near-infrared
light. To investigate the thermal stability of M2, TGA and DSC measurements were
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1
taken at a scan rate of 10°C · min under a nitrogen atmosphere. As shown in Figure 6
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Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
(c) and Table 1, M2 has good thermal stability and the 5% decomposition temperature
(
T5%) in nitrogen is higher than 250°C. The polymer has a lower glass transition
temperature; this is likely due to the low transition temperature of the polysiloxane
system itself. The GPC data of M2 are shown in Table 1 and Figure s2.
Figure 6. (a) Synthesis of the postfunctionalization by addition of F4-TCNQ for M1; (b)
Normalized UV-vis spectra of M1 and M2; (c) TGA of M2 at the heating rate of
-
1
1
0 °C · min under flowing nitrogen.
Table 1 Summary of molecular weights and thermal properties
a
a
a
b
c
c
polymer
Mn
Mw
Mw/Mn
Tg /°C
T5% /°C
T10% /°C
M2
3500
4600
1.31
34
231
298
a
Determined by GPC in THF using polystyrene standards
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Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
b
Glass transition temperature determined by the second heating scan of DSC
-
1
measurements at the scanning rate of 10 °C · min
c
5
% and 10% weight loss temperatures determined by TGA at the heating rate of
-
1
1
0 °C · min
3
.3 Photo-responsive characterization
For practical applications, thermal tuning is quite complicated and difficult. Using the
CLC mixture, it was demonstrated that the goal can also be accomplished with NIR
light. As shown in Figure 7 (a), a polyethylene terephthalate (PET) film was used as a
base material to make cells flexible. The prepared CLC mixture (93 wt% SLC1717, 6
wt% CD, 1 wt% M2) was injected into two PET films. Prior to irradiation, the cells
were purple at room temperature (33°C). When the cells were exposed to 808 nm NIR
-
2
light at an intensity of 50 mW · cm , the color of the irradiated portion rapidly changed.
This shows the feasibility of near-infrared light tuning. Cholesteric liquid crystals
(CLC) have a certain sensitivity to temperature, but this sensitivity is not obvious and
requires high temperature and response time. The addition of the near-infrared dye M2
and the temperature-sensitive chiral dopant CD alters the temperature sensitivity of the
CLC mixture and promotes the photothermal effect of the CLC system.
To further demonstrate the thermal effect of the CLC mixture, a thermal imaging test
was performed at room temperature. As shown in Figure 7 (b), the CLC mixture was
injected into the liquid crystal cell and handheld thermal imager was used to
photothermize the liquid crystal cell. In the absence of NIR light, the thermal imager
showed no heat absorption in the cells. When the cell was illuminated with an 808 nm
infrared laser, there was a significant hot spot in the cell thermography photograph,
demonstrating the great photothermal sensitivity of the CLC mixture. Due to the strong
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Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
absorption of near-infrared light by M2, the temperature in the vicinity of the light
irradiation is remarkably increased.
Figure 7 (c) is a polarized photograph of the CLC mixture in a liquid crystal cell with
different near-infrared illumination time. The inner surfaces of all liquid crystal cells are
oriented with PVA solution. It can be seen from the polarized photo that dye M2 and
chiral dopant CD have good solubility in the CLC system, and the system still exhibits a
relatively stable planar texture as increasing illumination time. SLC1717 has a high
phase transition temperature that allows us to maintain a stable liquid crystal system in
the near-infrared external illumination. As shown in Figure 7 (d), the LC phase of the
CD/SLC1717/M2 mixture was characterized using DSC, and only one exothermic peak
belonging to the conversion of the cholesterol phase to the isotropic phase was observed
above 80 °C during heating and cooling. This indicates that the liquid crystal system has
a high definition bright spot and the CLC system can be maintained for a long time
under illumination conditions.
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Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
Figure 7. (a) Real cell images created by NIR control of the helices in the CLC to show
patterns of RGB colors with a black background in 5 µm thick PET cells (1.0 cm × 1.0
cm); (b) Schematic diagram of thermal imaging of CLC mixture before and after NIR
illumination; (c) POM pictures of CLC mixture at different illumination times from 1s
to 8 s with a 100-fold objective lens; (d) DSC curve of the M2/CD/SLC1717 mixture at
-
1
a 5°C · min cooling rate.
Encouraged by the appropriate HTPs remarkable variation, the selective reflection of
CLC in visible spectrum region was demonstrated. CLC mixture was capillary filled
into a 5 µm thick PET cell that was black background on one side to create a contrast
with a bright iridescent Bragg reflective color from the CLC when viewed from the
other side of the cell. The reflection color of 6 wt% chiral dopant CD in SLC1717 was
dark purple at the cell and was tuned from blue to green and yellow upon 808 nm light
−2
irradiation (50 mW · cm ) in 8 s (Figure 8a). The whole process responds quickly, and
after the light stops, the color of the cells can be restored to dark purple.
HTP value of chiral dopant CD with temperature can be calculated from the
relationship between CLC reflection locations and temperature. As can be seen from
Figure 8 (b), the weight concentrations of the chiral dopant CD is 6 wt% and the
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Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
transmission peaks locate at 420 nm, 440 nm, 460 nm, 476 nm, 486 nm and 508 nm
when the temperature of the chiral dopant are 30, 40, 50, 60, 70 and 80°C, respectively.
The transmittances of curves within the bandwidth are both close to 45%. With the
increasing temperature of chiral dopant CD in the LC host of SLC-1717, the regions of
transmission shift to longer wavelengths. The HTP value of the chiral dopant can be
calculated according to Equations (1) and (2):
푝 = [(퐻푇푃) ∙ 푋푐]−1
λ = 푛 ∙ 푝
(1)
(2)
Here, p is the pitch of Ch-LC. Xc is the weight concentration of chiral dopant. λ is the
2
7
reflection bandwidth. n is the average birefringence of the LC. The results are shown
in figure 8(c). The results show that the helical twisting force of the chiral dopant CD
decreases with increasing temperature; the pitch of the CLC system is increasing and
the transmission curve is red-shifted.
Figure 8 (d) shows that the reflective notch of the CLC system varies with increasing
-
2
8
08 nm NIR (50 mW · cm ) illumination time. The extended illumination causes the
reflective notch to move toward longer wavelengths due to the strong absorption of
near-infrared light by M2; the reduction of the twisting force of the CD helix caused by
the conversion of NIR light into heat. The photothermal tuning range is in the range of
4
00-550 nm over the entire illumination time range. The reversibility and repeatability
of the light responsive properties have been proved. The transmission pattern of the
sample after 50 cycles of exposure and the relationship between transmission and time
after the sample was cooled were measured. The results show that the test samples have
good reversibility and repeatability. The results are shown in support information.
(Figure s3）
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Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
Figure 8. (a) Reflection color images of 1% M2 in LC host SLC1717 in a 5 µm thick
−2
PET cell (1mm x 1mm) upon exposure to 808 nm light (50 mW · cm ); (b) The CLC
cell (CD 6 wt%) transmission spectra as functions of temperature; (c) HTP value of
chiral dopant CD with temperature; (d) The CLC cell transmission spectra of 808 nm
light at different irradiation times.
Here, we draw a schematic diagram of the CLC system, as shown in Figure 9. The
pitch of the portion of the cholesteric liquid crystal in the system that is irradiated with
near-infrared light becomes large. Based on the selective reflection of cholesteric liquid
crystals, the wavelength of the reflected light changes that in turn shows a different
color. This is because the absorption of near-infrared light by M2 causes the local
temperature of the CLC system to rise, so that the helical twisting force of the CD
2
2

Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
decreases and the pitch of the system becomes larger. Different color displays can be
obtained by adjusting the NIR illumination time.
Figure 9. Schematic illustration of the self‐ organized helical superstructures in the
cholesteric LCs tuned by NIR light.
4
. Conclusions
In summary, it is feasible to control the reflective notch of the CLC system by NIR
light. We designed and synthesized the near-infrared absorbing dye M2 by double-click
reaction. M2 has good solubility and strong near-infrared absorption characteristics in a
liquid crystal system. The entire CLC system produces intense photothermal effects
through near-infrared illumination and has a better stability. The thermal effect of
excitation by near-infrared light changes the helical twisting force of the CD to cause
selective emission of the CLC system. This work has proven to be fast and efficient and
provides a new idea for the design of infrared absorbing dyes and the future application
of display and inspection systems.
Acknowledgements
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Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
This work was supported by the Fundamental Research Funds for the Central
Universities (Grant No. FRF-GF-19-003B), the National Natural Science Foundation of
China (Grant No. 51673157, 51773017, 51973017, 51673023), the State Key
Laboratory for Advanced Metals and Materials (No. 2018Z-06).
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Author statement
Zhitao Li: Conceptualization, Methodology. Validation, Investigation, Writing -
Original Draft, Data Curation
Dong Wang: Funding acquisition, Writing - Review & Editing, Resources
Hong Gao: Project administration
Hui Cao: Supervision, Writing - Review & Editing
Yuzhen Zhao: Project administration
Zongcheng Miao: Visualization
Zhou Yang: Project administration, Resources
Wanli He: Project administration, Resources
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Application of double clickJt oo up rr en paa lr eP nr ee a- pr - ri no f ro afred absorbing dye for photo-
thermal tuning of cholesteric liquid crystal
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.
☐
The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
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