Optical Behaviors of Cholesteric Liquid-Crystalline Polyester Composites with Various Chiral Photochromic Dopants

Article abstract of DOI:10.1021/acs.langmuir.5b03201

New developments in the field of chiral nematic liquid crystals, such as color displays, are now being widely proposed. This article describes the tunable incident reflection band based on composite materials of low-molecular-weight chiroptical dopants an

Full text of DOI:10.1021/acs.langmuir.5b03201

Article
pubs.acs.org/Langmuir
Optical Behaviors of Cholesteric Liquid-Crystalline Polyester
Composites with Various Chiral Photochromic Dopants
Chih-Chieh Chien and Jui-Hsiang Liu*
Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan
S
* Supporting Information
ABSTRACT: New developments in the field of chiral nematic liquid crystals, such as color
displays, are now being widely proposed. This article describes the tunable incident
reflection band based on composite materials of low-molecular-weight chiroptical dopants
and polymeric networks. These materials have advantages including easily manageable
color according to a change in the helical pitch of the cholesteric liquid crystal upon
exposure to light. A series of novel chiral dopants of isosorbide derivatives containing
photochromic groups and three new main-chain liquid crystalline polyesters were
synthesized and identified using nuclear magnetic resonance (NMR), Fourier transform
infrared spectroscopy (FTIR), and elemental analyses. The phase-transition temperatures
and the liquid-crystal phase determination of the synthesized polymers were estimated
using DSC, WAXD, and POM analyses. The influence of the dopant concentrations and
the solubility in a liquid crystalline polymer blend were also studied. The reflection band of
the cholesteric liquid crystalline composites could be adjusted and tuned with a wide range
of color variation across the entire visible region. A real image recording of the chiral
photochromic liquid crystalline polymer blend was achieved by exposing it to UV light through a mask.
main goal in preparing photochromic liquid crystals (LCs) is to
INTRODUCTION
■
obtain a material that responds to light such that the chemical
and physical properties are affected during photochemical
switching of the liquid-crystal phases. There are at least two
principal methods to obtain photosensitive LC polymers,
including (1) polymers with photosensitive chromophores in
side- or main-chain LCPs or (2) polymer mixtures doped with
photoresponsive compounds (photochromic dopants). The
latter case allows us to control the properties of the guest
photochromic dopant by light irradiation without any changes
in the polymer matrix itself. The photochromic groups play the
role of “switchers” in a liquid crystal, which can induce large
changes in their configuration and conformation upon
photoisomerization.14 These changes in the shape of the
photochromic groups lead to distortion of the packing of the
LC molecules, simultaneously inducing a local phase transition.
It is also well known that adding chiral molecules into an
achiral nematic LC host can induce the twisted molecular
arrangement to form a self-organized, optically tunable helical
structure.15−20 Photochemically tunable cholesteric liquid
crystals are the most attractive materials. The optical switching
of photochromism in cholesteric liquid crystal phases leads to
various effects not only on the liquid-crystal phase but also on
the helical structure. Adjustment of the cholesteric pitch of the
helical structure by light can be utilized in the practical
applications of optically addressable displays, recording
Systematic studies on main-chain liquid-crystalline polymers
(LCPs) did not begin until the mid-1970s.1 Main-chain LCPs
that have repeating rigid mesogenic units in the long main
chains, such as benzene rings interlinked at the para position,
tend to be infusible crystalline solids.2 These types of main-
chain polymers usually show quite high melting temperatures
above the decomposition temperature and are thus observed to
exhibit only lyotropic behavior. The transition temperature of
main-chain LCPs can be reduced by structural modifications,
such as through the disruption of the perfect regularity and
rigidity of the molecular chain by introducing flexible segments
or rigid kinks into the straight main chains.3 The most
commonly used connecting units joining the rigid core to the
disruptors are ester and ether groups. The other strategy is to
add a substituent in the aromatic ring, causing a decrease in the
crystallinity and lowering the melting point of the polyester.3 In
these semiflexible main-chain LCPs, the existence of nematic,
cholesteric, and smectic phases has been found.
The synthesis of main-chain LCPs normally follows step-
growth polymerization techniques, such as polycondensation.
Among main-chain LCPs, polyesters are preferred systems
because high-molecular-weight polymers can be easily obtained
by standard processes. Polyesters are defined as polymers
containing at least one ester linkage group per repeating unit.
The important properties of polyesters include melt process-
ability, outstanding thermal and chemical resistance,and
excellent mechanical properties.
Received: May 3, 2015
Recently, many photoinduced chiral systems have been
Revised: November 17, 2015
studied from experimental and theoretical viewpoints.4−13 The
© XXXX American Chemical Society
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Figure 1. Chemical structures of the chiral dopants D1, D2, and D3 with their transition temperatures and specific rotations. Abbreviations: K =
crystalline state, I = isotropic state.
Figure 2. Variations in the UV−vis absorption spectra of the chiroptical dopants (a) D1 and (b) D2 upon 254 nm UV irradiation.
materials, and photonic crystals.21 For example, cholesteric
color filters that can reveal reflected colors of red, green, and
RESULTS AND DISCUSSION
■
Synthesis of Chiroptical Dopants (D1, D2, and D3). As
blue via a patterned mask are a good application of the
illustrated in the Supporting Information Scheme S1, three
principle.22 Furthermore, the effects of tuning helical twisting
chiroptical dopants (abbreviated as D1, D2, and D3, Figure 1)
power of isosorbide-based chiral dopants on liquid crystals were
reported.23−25
containing photochromic azobenzene or cinnamoyl groups and
isosorbide as a chiral center were newly designed and
synthesized. The detailed synthetic procedures are explained
in the Experimental Section of this paper. The chemical
structures and the intermediates were confirmed by FT-IR, H
NMR (see Figures S1−S3 in the Supporting Information), and
elemental analyses.
Photochemistry of Azobenzene- or Cinnamate-Func-
tionalized Chiroptical Dopants (D1, D2, and D3). In this
research, the chiral dopants D1 and D2 containing photo-
isomerizable cinnamoyl groups and D3 containing azobenzene
groups were synthesized. To confirm the photoreactivities of
the photoisomerizable groups of the synthesized chiral dopants,
we investigated the UV absorption spectra of D1, D2, and D3
as a function of the UV exposure time.
The chiral dopant D1 showed a maximum absorption at
∼314 nm in a chloroform solution (5 × 10−5 M), as seen in
Figure 2a. This absorption was attributed to the π−π*
transition of the E,E-form cinnamoyl groups. When the sample
solution was exposed to 254 nm light, the π−π* absorbance
decreased depending on the increase in the UV irradiation time.
The most straightforward approach to develop such a photo-
optical material would be to synthesize a “hybrid” molecule
composed of chiral and photochromic moieties. Here we report
the design and synthesis of three new chiral dopants containing
photochromic azobenzene or cinnamate with an isosorbide
moiety situated at the center of the molecules. As described in
Figure 1, the molecular structures of the chiroptical dopants
were composed of two different groups: on the two sides,
photochromic groups could reversibly respond to lights, and at
the center, the presence of an isosorbide group may provide
molecular chirality and induce the helix packing. We also
investigated the thermal and photochemical behaviors of the
photochromic chiral dopants in the synthesized main-chain
liquid-crystalline polymer systems. Main-chain liquid-crystalline
polymers with and without isosorbide chiral diol were
synthesized. The influence of the dopant concentration and
the solubility in the main-chain liquid crystalline polymer
mixture were also studied.
1
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Figure 3. (a) Dependence of the UV−vis absorption spectra of D3 on the irradiation time under 365 nm irradiation and (b) recovery of the UV−vis
spectra of D3 in the dark.
The photochemical reaction reached a photostationary state
after a 180 s irradiation, holding two isosbestic points at 268
and 342 nm. The presence of the isosbestic points during the
photochemical reaction suggested that E-Z photoisomerization
was the predominant photoreaction in the solution of the chiral
dopant D1.26 Although most E-Z photoisomerizations are
reversible, the Z to E relaxation of D1 with cinnamate moieties
was an irreversible reaction due to the high activation barrier
between the two isomers.27 The photoisomerization of the
chiroptical dopant D1 can be seen in the Supporting
Information Figure S4.
Figure 2b shows the photoreactivity of the chiral dopant D2
in a CHCl3 solution (5 × 10−5 M). From the UV absorption
spectra, the maximum peak intensity was observed at 288 nm,
which was the characteristic peak of the cinnamate moiety; the
peak decreased as the UV exposure dose increased. The
decrease in the peak originated from the E-Z photoisomeriza-
tion of the attached cinnamoyl groups of D2. From the UV
absorption spectra, it was found that the E-Z photoisomeriza-
tion of the cinnamate moiety of D2 successfully occurred due
to the UV exposure. The successive spectral changes in the
absorption spectra of D2 were characterized by two isosbestic
points at 262 and 318 nm before 20 min of irradiation;
however, further exposure led to the disappearance of the
isosbestic points, which indicated that not only E-Z isomer-
ization but also another photochromic process occurred. In the
case of aryl esters of cinnamic acid, the photodimerization
reaction simultaneously occurred with a photo-Fries rearrange-
ment reaction.28 The schematic of the photo-Fries rearrange-
ment of the aryl ester of the cinnamic acid derivative under UV
irradiation is shown in Figure S5. Moreover, the absorption
bands at approximately 318 to 400 nm slightly increased during
the UV irradiation, which was also attributed to the photo-Fries
rearrangement of the cinnamate moieties.
decreased depending on the increase in the UV irradiation time,
whereas the n−π* band slightly increased in absorbance via E-Z
photoisomerization (Figure 3a). A photostationary state (PSS)
was reached after irradiation at 365 nm for ∼6 min. The
absorption stability of the chiral dopant D3 in the dark is
shown in Figure 3b. Theoretically, the E form of azobenzene is
more stable than the Z form, and the bent Z form may
gradually return to the linear E form, even in the dark.30 After 4
days under dark conditions, the absorption band returned to its
initial state, which was attributed to the Z to E photo-
isomerization, revealing that the E-Z photoisomerization of the
azobenzene-functionalized chiroptical dopant D3 was a
reversible photochemical reaction. The slow Z-E reversion of
D3 might be due to the effects of the substituents of
azobenzene on the molecular interaction, leading to variations
in the speed of the configuration change.
Synthesis of Main-Chain Liquid-Crystalline Polyesters.
Various approaches have been successfully carried out for the
synthesis of polyesters, which have usually been prepared by
solution or interfacial polycondensation and also by the ester
exchange reaction at high temperature or reduced pressure. In
these usual techniques, monomers should be prepared before
polymerization.31−33 Direct polycondensation of free carboxylic
acids and aromatic diols is a more useful technique because it
can be operative under mild conditions. A series of novel
polyesters was obtained by direct step growth polymerization
(direct polycondensation) involving the use of dicarboxylic
acids and an aromatic diol under mild conditions. Three types
of diols with different mesogenic groups were employed. Two
diol monomers, methyl hydroquinone and 4,4′-bisphenol
(abbreviated as MHQ and 2B, respectively, Figure 4), were
commercially available. The synthetic routes of the dicarboxylic
acid (abbreviated as M1, Figure 4) and chiral-based diol
(abbreviated as M2, Figure 4) for the preparation of polyesters
are shown in Scheme 1. The detailed synthetic procedures are
explained in the Experimental Section. The chemical structures
and their intermediates were confirmed using FT-IR, 1H NMR
(see Figures S6 and S7 in the Supporting Information), and
elemental analyses.
The photochemical property of the chiral dopant D3
containing azobenzene groups was studied using UV−vis
spectra following by irradiation with 365 nm UV light. Figure
3 shows the steady-state ultraviolet−visible spectroscopy (UV−
vis) absorption spectra of D3 in a chloroform solution (5 ×
10−5 M). The spectral shapes were similar to each other and
had characteristics in common with isosorbide complexes that
contain azobenzene derivatives as a photochromic group.29 The
major absorption band at approximately 292 to 392 nm was
assigned to the π−π* transition of the E,E azobenzene moieties,
and a weak absorbance at ∼392 to 500 nm was ascribed to the
n-π* transition of Z,Z-form isomers. When the sample solution
was exposed to 365 nm light, the π−π* absorbance drastically
The polyesters were synthesized by copolymerizing a small
amount of the chiral-based diol M2 because isosorbide-
containing polyesters are known to possess a high helical
twisting power (HTP) and are effective for breaking the
molecular packing symmetries in a liquid-crystalline matrix.26 In
addition, methyl-substituted hydroquinone (MHQ) was
selected as a diol monomer because the substituted group is
useful for lowering the phase-transition temperature.34,35
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The synthesized polyesters with mesogenic units in the main
chain consisting of oxyethylene spacers showed liquid-crystal
phases. The flexibility of the carbon−oxygen bonds of the
oxyethylene spacers in the monomer M1 and the methyl-
substituted mesogenic core decreased the rigidity of these
moieties and increased the mobility between polymer chains,
resulting in a broadened liquid crystal phase and a low glass-
transition temperature (Tg).36−40
Except for P1, which displayed a nematic liquid crystal phase
(thread-like texture, Figure 6a), P2 and P3 showed planar and
oily streak textures under POM observation, which were the
typical textures of the cholesteric liquid-crystal phase, as
indicated in Figure 6b−d. This result indicated that the
cholesteric liquid-crystal phase could be induced by small
amounts of a chiral segment in main-chain polyesters.
Moreover, the addition of monomer bisphenol (2B) increased
the mesogen length, increased the phase-transition temper-
ature, and broadened the temperature range of the liquid-
crystal phase.38 As seen in the Supporting Information Table
S2, polymer P3 showed a much broader liquid-crystal phase
than polymers P1 and P2.
Figure S11 illustrates the WAXD powder patterns for P3
from 85 to 125 °C that were obtained during heating. At 85 °C,
an amorphous scattering halo in the high-angle region at ∼20°
corresponding to the liquid-like order in the lateral molecular
packing was observed. As the temperature increased to 95 °C, a
weak peak at 3.29° with a d spacing of 26.84 Å was detected in
the low-angle region. The WAXD patterns for the polymer P3
at 95 to 115 °C indicated a typical sematic LC phase, and the
calculated distance of the layer structure was 26.84 Å.39 In
addition, a slight broadening of this scattering halo occurred
with increasing temperature. The structural changes of P3 in
Figure S11 could be characterized by the d-spacing changes,
indicating that the phase transition occurred.37 The WAXD
results were identical to the DSC and POM results.
Figure 4. Chemical structures of the aromatic dicarboxylic acid M1
and the diols M2, MHQ, and 2B for the synthesis of aromatic
polyesters by direct step-growth polymerization (direct polycondensa-
tion).
The synthesis procedures and possible sequential structures
of the oxyethylene-based aliphatic/aromatic polyesters are
summarized in Figure 5 and Figures S8 and S9 in the
Supporting Information. The properties of the resulting
polyesters synthesized from different mixtures of diols are
summarized in the Supporting Information. In each synthesis
system with a specific diol, dicarboxylic acid (M1)/diol (MHQ,
MHQ+M2, or MHQ+M2+2B) feed molar ratios of 1:1 were
employed. These polymers had a soft segment, such as an
oxyethylene moiety, in the main chain to improve the solubility
in organic solvents in comparison with aromatic polyesters.
The actual methyl hydroquinone/M2 molar ratios in the
chain structure were calculated from the integration peaks of
CH3 in MHQ at 2.22 ppm and CH in M2 located in the 5.07
ppm region. The high yield, molecular weight, and moderately
broad polydispersity index indicated that the use of p-TsCl/Py/
DMF as a condensing agent for the synthesis of polyesters
could successfully induce effective direct polycondensation
between dicarboxylic acids and aromatic diols.
Thermal Characterization of the Polyesters. The
thermal properties and liquid-crystal phase behaviors of the
synthesized polymers were characterized by DSC, TGA, POM,
and WAXD. The phase-transition temperatures of the
polyesters were characterized by DSC, as shown in Figure
S10, and the liquid-crystal phases were determined by hot-stage
POM and WAXD. The results are summarized in the
Supporting Information Table S2.
The thermal stabilities and thermal degradation patterns of
the polymers were investigated using TGA measurements with
a heating rate of 20 °C/min under a nitrogen atmosphere. The
5
parameters of the 5% weight loss temperature (Td ), 50%
weight loss temperature (Td50), and residual weight are
summarized in the Supporting Information Table S2. The
results showed that the decomposition temperatures at 5%
weight loss were >350 °C, and the residues at 600 °C were
∼12−15 wt % for all samples, indicating that the synthesized
polyesters had high thermal stability. Polymer P3 had a distinct
5
thermal stability up to 388.5 °C (Td ), indicating that the
incorporation of 2B monomer into the polymer chain increased
a
Scheme 1. Synthesis of M1 and M2
a
Reagents and conditions: (i) p-toluenesulfonyl chloride, KOH, THF, 30 °C, 24 h; (ii) ethyl 4-hydroxybenzoate, K2CO3, 2-butanone, reflux, 16 h;
(iii) KOH, MeOH/H2O, reflux, 16 h; and (iv) p-toluenesulfonic acid monohydrate, xylene, 145 °C, 7 h.
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Figure 5. Synthesis route for the main-chain liquid crystalline polyester P1 using p-TsCl/Py/DMF as a condensing agent.
wt % D3/P1 system, indicating a lack of compatibility between
D3 and P1 and resulting in failure to induce the CLC phase. A
selective reflection in the visible-light region could be achieved
by adding a small amount of chiral dopant, indicating that the
isosorbide derivative dopant revealed high helical twisting
power (HTP) in the liquid crystalline matrix. The phototuning
behaviors of the mixtures D1/P1 and D2/P1 were essentially
identical. As seen in Figure 7, exposure to UV light caused a
shift in the reflection notch. Tuning bands from 200−250 nm
in the visible region were observed. The results provided clear
evidence of the alternation of the CLC pitch by the
photochemical isomerization of the chiral dopant. UV
irradiation of the mixture served to isomerize the cinnamoyl-
containing isosorbide molecule, affecting the pitch and resulting
in a red shift of the reflection wavelength across the visible
region. The phototuning rates of the photoresponsive CLC
materials were related to the ability of the generated Z-form
isomers to affect the pitch. As the fraction of the chiral dopant
was increased to 5 wt %, a clear increase in the tuning rate was
observed, as shown in the Supporting Information Figure S12.
The transmission spectra of the CLC mixtures of chiral
dopants in cholesteric polymer P2 both before and during
phototuning are shown in Figure 8. Here all of the systems (3
wt % of the chiral dopants D1, D2, and D3 in P2) were
induced to show the CLC phase. Although a phase separation
was brought about by mixing photoisomerizable chiral dopant
in the polymer system at a concentration >3 wt %, the high
HTP value of the isosorbide derivative dopant brought a high
efficiency of reflection light from near-infrared to visible region.
In these three systems, only D1/P2 showed a UV-tuning
property in the bulk condition. The selective reflections
remained in the initial positions for the D2/P2 and D3/P2
samples even after a long photoirradiation. This result could be
due to the high viscosity and rigidity of the main-chain
Figure 6. POM textures of (a) P1 at 120 °C, (b) P2 at 150 °C, and P3
at (c) 115 and (d) 160 °C (×400).
the rigidity of the polymer and offered a stabilizing effect
against decomposition.
Optical Behavior of Liquid-Crystalline Polyester
Composites. In our studies of the optical properties of the
systems of chiral photochromic dopant/liquid-crystalline
polymer composites, the synthesized compounds D1−D3
were used as photoresponsive chiral (chiroptical) dopants.
The photochemical transformations of selective light reflection
were studied at elevated temperatures (e.g., in the cholesteric
liquid crystal phase) and recorded using an USB-2000 fiber
optic spectrometer.
The dynamic optical responses of the nematic polymer P1
doped with different optically active dopants are demonstrated
in Figure 7. In the D1/P1 and D2/P1 systems, the cholesteric
liquid crystal (CLC) phase was induced at a concentration of 3
wt % addition; however, phase separation was observed in the 3
Figure 7. Effect of UV irradiation on the selective reflection bands of the 3 wt % (a) D1/P1 and (b) D2/P1 systems at 120 °C.
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Figure 8. Effect of UV irradiation on the selective reflection bands of (a) 3 wt % D1/P2 at 150 °C and (b) 3 wt % D2/P2 in the solution state. (c)
Dependence of the transmission spectrum of 3 wt % D3/P2 on UV tuning and thermal recovery at 150 °C. (d) Effects of chiral photochromic
dopants on the selective reflection band of P2.
polyester with higher chain entanglement and the distance
between the photosensitive and chiral segments. To overcome
this problem, the phototuning properties of the D2/P2 and
D3/P2 mixtures were studied under solution conditions in
chloroform.
150 °C (Figure 8c). The slow thermally induced relaxation
could be attributed to the effect of the polymer matrix
associated with the motion of the polyester polymer backbone.
As a result, a slow Z-E isomerization process of the entire
doped polymeric system could be the result of the slow
polymer chain rearrangement.44
Red shifts of the selective light reflection bands were
observed with an increase in the UV irradiation time for all
doped P2 systems. As shown in Figure 8a, the phototuning of
D1/P2 reached a maximum wavelength shift (over 300 nm
tuning range), at which point the reflection intensity decreased
until the samples were partially isotropic. The D2/P2 mixture
in the solution state was observed to have tuning of ∼150 nm
from exposure to UV light (shown in Figure 8b), which was
speculated to be primarily due to the reduction of the polymer
viscosity and the extension of the polymer chains in the solvent
environment, resulting in the easing tuning of the CLC pitch,
which was affected by the conformational change upon
generation of the Z-form isomer. There are many photo-
responsive chiral materials that have been synthesized and
studied that have been observed to reveal a transition from a
cholesteric to a photoinduced isotropic state in the photo-
isomerization of the host molecules upon UV irradiation.41−43
In our system, the main-chain CLC polymers showed lesser
influences of the light on the selective light reflection bands
compared with the referenced works. The materials showed
large-scale tuning as well as the maintenance of the CLC phase
upon UV irradiation. This result could be due to the high
viscosity of the main-chain polyester with higher chain
entanglement, resulting in the local mobility of the chains in
the helical structures.
The HTPs of the dopants D1, D2, and D3 in P2 before
irradiation (initial) were calculated according to Figure 8d. The
highest initial HTP of 126 μm−1 was obtained for D3, which
was larger than that for D1 and D2 in the same liquid-crystal
host (P2). The initial HTP of 126 μm−1 changed to 77 μm−1
upon 365 nm light irradiation. The changes in the helical pitch
and HTP were attributed to the conformational change
between E and Z isomerization of the azobenzene unit in the
chiral dopant by photoirradiation.
Figure 9 presents the phototuning properties of doped P3
CLC mixtures. The spectrum of the pure P3 polymer showed a
reflection band at 980 nm. The reflection notch of the pure P3
polymer was blue-shifted due to the addition of the chiral
dopants. The chiral dopants could exhibit a different ability to
twist the LC molecules, showing a different sense of rotation.
Compared with the commercially available chiral dopant R811,
the synthesized dopants were observed to reveal a marked blue
shift of the reflection band under the same concentration. The
result indicated that D3 showed a much more efficient ability to
induce CLC in the P3 system. It should be noted that the
ability to induce the helical arrangement of LC molecules is
related not only to the HTP of the chiral dopant itself but also
to the solute−solvent interactions between the guest dopant
(chiral solution) and the host polymer (LC solvent).45,46
To investigate the practical photochromism of the CLC
films, the mixture of the synthesized polymer P3 doped with
the chiroptical dopant D1 was coated on a glass substrate.
Visible-light reflection could be observed for a mixture of 6 wt
% D1 in P3 at the cholesteric mesophase. The initial spectrum
As has been discussed, azobenzene is a photoreversibility
photochromic molecule. In the 3 wt % D3/P3 solution system,
the initial peak of 3 wt % D3/P3 shifted from 423 to 695 nm
after UV irradiation and then showed a blue shift to 591 nm,
which was a thermally induced reaction after 3 h treatment at
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clearing temperature than that of D1 and D2. The results
suggest that molecular interaction between D3 molecules is
extremely tough, leading to the decrease in solubility of D3 in
both E7 and MJ05681. In the case of D3/P2 system,
intermolecular forces between D3 and P2 are compatible,
leading to the formation of CLC phases.
EXPERIMENTAL SECTION
■
Instruments. The Fourier transform infrared spectroscopy (FTIR)
spectra were recorded on a Jasco VALOR III Fourier transform
infrared spectrophotometer. Nuclear magnetic resonance (NMR)
spectra were obtained using a Bruker AMX-400 high-resolution NMR
spectrometer. Differential scanning calorimetry (DSC) was conducted
on a PerkinElmer DSC 7 in a nitrogen atmosphere with the heating
and cooling rates of +5 and −5 °C min−1, respectively. A
thermogravimetric analyzer (TGA-7) from PerkinElmer was used to
record the temperature of thermal decomposition at a heating rate of
+40 K min−1 under a nitrogen atmosphere. The cross-sectional
morphology of the samples was characterized by transmission electron
microscopy (TEM) using a JEOL JEM-1200CX-II microscope with a
Cu-grid substrate. The anisotropic properties of the highly ordered,
self-assembled samples were investigated using an Olympus BH-2
polarized light microscope (POM) equipped with a Mettler hot stage
FP-82, and the temperature scanning rate was determined to be 10 °C
min−1. Photoisomerization studies were conducted using UV
irradiation with a UVP UVG-54 (254 nm, 6 W) and a UVP UVL-
56 (365 nm, 6 W) as the light sources. Samples were irradiated with a
distance of 3 cm from UV-lamp. E-Z photochemical isomerizations
were investigated using the UV/vis absorption spectra measured using
a Jasco V-550 spectrophotometer. The synthesized compounds and
Figure 9. Effect of chiral dopants on the selective reflection band of
P3.
before photoirradiation showed a reflection band at 452 nm
(Figure 10a) with a blue color, as shown in Figure 10b. UV
1
polymers were identified using FTIR and H NMR.
Synthesis of Isosorbide 2,5-Bis(4-methoxycinnamate) (D1).
4-Methoxycinnamic acid (3.56 g, 20 mmol) and isosorbide (1.46 g, 10
mmol) were dissolved in 60 mL of CH2Cl2, and the mixture was
stirred at 30 °C. N,N′-Dicyclohexyl carbodiimide (3.09 g, 15 mmol)
and 4-dimethylaminopyridine (0.12 g, 1 mmol) were dissolved in 40
mL of CH2Cl2 and dripped into the vigorously stirring solution. The
reaction mixture was stirred for 48 h at 30 °C. After filtering out the
urea solid that formed, the solvent was removed. The remaining
residue was dissolved in CH2Cl2 and washed with distilled water. After
drying the organic layer over MgSO4, the product was purified by silica
gel column chromatography using ethyl acetate/n-hexane (1/5) as the
eluent.
Synthesis of 4-(Methoxycarbonyloxy) Cinnamic Acid (1).
Trans-4-hydroxycinnamic acid (8.21 g, 50 mmol) and NaOH (6.00 g,
150 mmol) were dissolved in 150 mL of H2O and stirred for 1 h at 0
°C. Methyl chloroformate (8.03 g, 85 mmol) was further added
dropwise, and the mixture was stirred for 4 h at room temperature.
After completing the reaction, the resulting solution was poured in
water and acidified with diluted HCl until weakly acidic. The residue
was washed with distilled water twice and recrystallized with ethanol to
give the product in 91.7% yield.
Synthesis of Isosorbide 2,5-Bis(4-methoxycarbonyloxy
cinnamate) (D2). The mixture of the intermediate 1 (8.00 g, 36
mmol), DMF, and 30 mL of SOCl2 was heated at reflux for 6 h. After
the reaction, SOCl2 was removed, and the crude product was dissolved
in 70 mL of anhydrous THF. The solution was added dropwise to an
anhydrous THF solution of isosorbide 2,5-bis (4-hydroxybenzoate)
(M2) (6.59 g, 17 mmol) and triethylamine (4.05 g, 40 mmol) under a
nitrogen atmosphere at 0 °C. The reaction mixture was stirred for 72 h
at 30 °C. After filtering out the precipitate solid that formed, the
solvent was removed. The remaining residue was dissolved in CHCl3
and washed with distilled water. The organic layer was dried over
MgSO4. After removing the solvent, the residue was recrystallized with
ethanol to give the product in 13.3% yield.
Figure 10. (a) Effect of UV irradiation on the reflection band of the 6
wt % D1/P3 system at 200 °C. (b) Recorded color pattern using the 6
wt % D1/P3 composite at 200 °C with various UV irradiation times.
irradiation of the sample resulted in a shift of the reflection
band toward longer wavelengths. The photoinduced switch
reached steady state by increasing the UV irradiation time to 20
min. The reflection band shifted to 715 nm, and the reflection
color changed to red from green. This system exhibited a wide
range of color variation across the entire visible region with
exposure to light. This result revealed that the dopant
concentration of 6 wt % was sufficient to induce photocontrol
of the RGB reflection colors. The efficiency was much more
sensitive compared with those of reported chiral photochromic
dopants so far.47,48
To correlate the synthesized dopant D3 to other published
papers, commercially available E7 and MJ05681 nematic liquid
crystals purchased from Merck were used for the evaluation of
HTP- of D3-induced cells. Unfortunately, it was found that D3
is insoluble in both E7 and MJ05681 nematic liquid crystals. It
is ascribed to molecular structure and intermolecular forces
difference. As shown in Figure 1, D3 reveals much higher
Synthesis of Isosorbide Bis(4-nitrobenzoat) (2). Isosorbide
(3.65 g, 25 mmol) and triethylamine (5.77 g, 57 mmol) were dissolved
in 20 mL of CHCl3, and the mixture was stirred at 0 °C. A solution of
4-nitrobenzoyl chloride (9.28 g, 50 mmol) dissolved in 20 mL of
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CHCl3 was further added dropwise, and the mixture was stirred for 6 h
at 30 °C. After completing the reaction, the resulting solution was
poured in water and extracted by CHCl3. The extracted solution was
concentrated using a rotary evaporator under vacuum. The residue was
recrystallized with ethanol to give the product in 29.9% yield.
Synthesis of Isosorbide Bis(4-aminobenzoat) (3). The
intermediate 2 (4.70 g, 11 mmol) and ammonium chloride (4.50 g,
84 mmol) were dissolved in 50 mL of H2O and 80 mL of ethanol. A
solution of sodium sulfide hydrate (14.40 g, 60 mmol) was further
added to the mixture and then heated at reflux for 6 h. After cooling to
room temperature, the precipitated solid was collected. The solid was
redissolved in an aqueous HCl (10 M) solution (100 mL) and filtered.
After ammonium was added to the solution to give pH 8, the
precipitated solid was collected and dried. The residue was
recrystallized with ethanol to give the product in 48.4% yield.
Synthesis of Isosorbide Bis(4-hydroxy-4′-azobenzoat) (4).
The intermediate 3 (0.7 g (1.8 mmol) was dissolved in a mixture of 6
mL of concentrated hydrochloric acid and water. With stirring at 0 °C,
0.4 g (5.8 mmol) of sodium nitrite in water (4 mL) was added to the
solution to produce diazonium salt. After the resulting solution was
stirred at 0 °C for 1 h, a mixture of phenol (0.55 g, 5.8 mmol) and
sodium hydroxide (0.53 g, 13.3 mmol) in 10 mL of water cooled to 0
°C was slowly added. The reaction mixture was stirred at 0 °C for 2 h.
After HCl (5%) was added to the reaction mixture to give pH 4, the
precipitated solid was collected and dried. The residue was
recrystallized with ethanol to give the product in 25% yield.
Synthesis of Isosorbide Bis(4-octoxycarbonyloxy-4′-azo-
benzoat) (D3). Nonanoyl chloride (0.97 g, 5.5 mmol) was added
dropwise to a solution of a mixture of the intermediate 4 (1.5 g, 2.5
mmol) and triethylamine (0.58 g, 5.75 mmol) in anhydrous THF (40
mL). The reaction mixture was stirred at room temperature for 24 h.
After completing the reaction, the resulting solution was poured in
distilled water and extracted by CHCl3. The extracted solution was
concentrated using a rotary evaporator under vacuum. The residue was
recrystallized with THF to give the product in 78% yield.
Synthesis of Tetraethylene Glycol Ditosylate (5). Tetra-
ethylene glycol (11.8 g, 61 mmol) and p-toluenesulfonyl chloride
(34.5 g, 182 mmol) were dissolved in 90 mL of THF in an ice bath. An
aqueous solution of KOH (16 M, 24 mL) was further added dropwise,
and the mixture was stirred for 24 h at room temperature. After
completing the reaction, the resulting solution was poured in ice water
and extracted by CHCl3. The extracted solution was concentrated
using a rotary evaporator under vacuum. The product was purified by
silica gel column chromatography using ethyl acetate/n-hexane (1/1)
as the eluent to give the product in 95.1% yield.
Synthesis of Tetraethylene Glycol Bis(4-carboxyphenyl)
Ether (M1). A solution of ethyl 4-hydroxybenzoate (18.3 g, 110
mmol), K2CO3 (55.3 g, 400 mmol), and KI (3.0 g, 18 mmol) in 300
mL of 2-butanone was heated at reflux for 2 h under a nitrogen
atmosphere. A solution of 5 (27.6 g) in 30 mL of 2-butanone was
further added dropwise and heated to reflux for 16 h. The excess
K2CO3 was filtered off from the hot solution. The solvent was
removed under reduced pressure. The product was redissolved in 300
mL of methanol, and a 30 mL aqueous solution of KOH (22.4 g, 400
mmol) was further added to the product. The mixture was heated to
reflux for 16 h and then neutralized with conc. HCl to obtain a
precipitate. The precipitate was filtered, and the filtrate was washed
with hot water and then dried under vacuum. The residue was
recrystallized with ethanol to give the product in 82.3% yield.
Synthesis of Isosorbide 2,5-Bis(4-hydroxybenzoate) (M2). A
mixture of 4-hydroxybenzoic acid (16.0 g, 116 mmol), isosorbide (8.0
g, 55 mmol), and p-toluenesulfonic acid monohydrate (0.44 g, 2.4
mmol) in 100 mL of xylenes was placed in a flask equipped with a
Dean−Stark trap, a condenser, and a mechanical stirrer. The reaction
mixture was heated to 145 °C for 7 h, at which time an additional
charge of p-toluenesulfonic acid (0.22 g, 1.2 mmol) was added, and the
mixture was heated back to reflux. After 3 h, the reaction was allowed
to cool to room temperature. The solvent was removed, and remaining
residue was dissolved in 100 mL of ethyl acetate and washed twice
with a 1% (w/v) sodium bicarbonate solution. The solvent was
removed under reduced pressure. The product was purified by silica
gel column chromatography using ethyl acetate/n-hexane (1/4) as the
eluent to give the product in 91.2% yield.
Synthesis of Main-Chain Liquid Crystalline Polyesters. In this
study, the liquid-crystalline polyesters containing various proportions
of different diols were prepared using the following procedure. For the
synthesis of P1, p-toluenesulfonyl chloride (p-TsCl) (1.27 g, 6.7
mmol), N,N-dimethylformamide (DMF) (0.11 g, 1.5 mmol), and 5
mL of anhydrous pyridine (Py) were stirred in a two-necked round-
bottomed flask at room temperature for 30 min under a nitrogen
atmosphere. The compound M1 (1.00 g, 2.30 mmol) in 5 mL of
anhydrous pyridine was added to the mixture. The reaction mixture
was stirred at room temperature for 10 min and then stirred at 120 °C
for 10 min. Methyl hydroquinone (MHQ) (0.28 g, 2.23 mmol)
dissolved in anhydrous pyridine was then added to the mixture and
refluxed for 3 h. After the reaction, the mixture was poured in 300 mL
of methanol to precipitate the polymer. The polymer was dissolved in
chloroform and reprecipitated with methanol twice. The obtained
polymer P1 was dried at 70 °C under vacuum for 12 h. The polymers
P2 and P3 were synthesized using the same procedure as polymer P1.
CONCLUSIONS
■
The polymerizable monomer dicarboxylic acid M1, the chiral
diol M2, and a series of chiroptical dopants (D1, D2, and D3)
were synthesized and identified using NMR, FTIR, and
elemental analyses. The synthesized main-chain polyesters
were found to exhibit nematic, smectic, or cholesteric liquid-
crystal phases. The results from the transmission spectra
revealed that the synthesized chiroptical dopants had high HTP
values and good compatibilities with the cholesteric liquid-
crystalline polymer P3. Stable colorful reflective patterns could
be obtained with large-scale tuning through the entire visible
region as well as the maintenance of the CLC phase upon UV
irradiation. The discovered phenomena suggest the potential
use of chiral photochromic dopant/liquid-crystalline polymer
composites as UV-light-controllable RGB materials.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.lang-
muir.5b03201.
1H NMR spectra of D1−D3; photoisomerization of the
chiroptical dopant D1; photo-fries rearrangement of the
aryl ester of the cinnamic acid derivative; 1H NMR
spectra of M1 and M2; synthesis route for the main-
chain liquid-crystalline polyesters P2 and P3 using p-
TsCl/Py/DMF as a condensing agent; DSC curves of
P1, P2, and P3; Synthesis of D1, D2, and D3; results of
polymerization; WAXD patterns of P3 at a heating rate
of 10 °C/min at various temperatures; comparison of the
shift in the reflection notch versus the irradiation time of
the D1/P1 and D2/P1 CLC mixtures doped with
different ratios of the chiroptical dopants; and phase
transition temperature and thermal resistance of
polymers. (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jhliu@mail.ncku.edu.tw.
Notes
The authors declare no competing financial interest.
H
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(20) Li, Q.; Green, L.; Venkataraman, N.; Shiyanovskaya, I.; Khan,
A.; Urbas, A.; Doane, J. W. Reversible Photoswitchable Axially Chiral
Dopants with High Helical Twisting Power. J. Am. Chem. Soc. 2007,
129, 12908−12909.
ACKNOWLEDGMENTS
■
We thank the National Science Council (NSC) of the Republic
of China (Taiwan) for financially supporting this research
under Contract No. NSC 101-2221-E-006-074-MY3. This
research was, in part, supported by the Ministry of Education,
Taiwan, R.O.C., The Aim for the Top University Project to the
National Cheng Kung University.
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