Color tunable novel amphotropic liquid crystalline acrylate end-capped with a cholesteryl group

Article abstract of DOI:10.1246/cl.2010.485

We report a novel, monomelic, lyotropic, cholesteric liquid crystal revealing selective reflection in the visible light region and having photochromic properties in the lyotropic liquid crystal phase. Monomer (+)-cholesteryl (6-acryloyloxyhexyloxy)-4-cinnamate was synthesized. Liquid crystal phases of Ml were identified using polarized optical microscopic and X-ray diffraction methods. Reflection light spectra were analyzed using an optical fiber system.

Full text of DOI:10.1246/cl.2010.485

doi:10.1246/cl.2010.485
Published on the web April 3, 2010
485
Color Tunable Novel Amphotropic Liquid Crystalline Acrylate End-capped
with a Cholesteryl Group
Feng-Min Hsieh, Hau Fang, and Jui-Hsiang Liu*
Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan 70101
(Received February 18, 2010; CL-100167; E-mail: jhliu@mail.ncku.edu.tw)
We report a novel, monomeric, lyotropic, cholesteric liquid
crystal revealing selective reflection in the visible light region
and having photochromic properties in the lyotropic liquid
crystal phase. Monomer (+)-cholesteryl (6-acryloyloxyhexyl-
oxy)-4-cinnamate was synthesized. Liquid crystal phases of M1
were identified using polarized optical microscopic and X-ray
diffraction methods. Reflection light spectra were analyzed using
an optical fiber system.
polyamides, polyisocyanates, polycarbodiimides, or polygluta-
mates. Thus far, only a few attempts have been made to produce
lyotropic cholesteric liquid crystals with selective reflection in
the visible light region.5 To our knowledge, this work is the first
report of a small molecular system of lyotropic cholesteric liquid
crystals revealing selective reflection in the visible light region
and of photochromic properties in lyotropic liquid crystals in
general.
In this paper, a novel monomeric (+)-cholesteryl (6-
acryloyloxyhexyloxy)-4-cinnamate (M1) with a photochromic
cinnamoyl functional group was synthesized, and the ampho-
tropic properties of the monomer were investigated. The pure
monomer M1 exhibited a thermotropic cholesteric liquid crystal
phase, with a phase transition temperature of K 74 Ch 84 I.
Dissolving this monomeric liquid crystal in the isotropic solvent
dichloromethane (CH2Cl2) led to the formation of a lyotropic
liquid crystal phase. Interestingly, the lyotropic liquid crystal cell
revealed significant selective light reflection in the visible region.
The molecular structure of the synthesized novel monomer
(M1) is shown in Scheme 1, where compound 1 was synthe-
sized by following procedures described in our previous report.6
Compound 1 (8 g, 0.025 mol), cholesterol (10.7 g, 0.028 mol),
and DMAP (0.61 g, 0.005 mol) were dissolved in 50 mL of
CH2Cl2. DCC (6.23 g, 0.03 mol) dissolved in 25 mL of CH2Cl2
was added to the mixture then was stirred for 48 h at 30 °C. The
resulting solution was filtered and the precipitated urea was
removed, washed twice with 0.5 M HCl and then with saturated
aqueous NaHCO3 solution, evaporated, and dried in vacuum.
The crude product was purified using column chromatography
(silica gel, ethyl acetate/hexane = 1/2) and then recrystallized
from the mixture of ethyl acetate/hexane (1/2). White crystals
of M1 were obtained. Yield: 72.3%. FT-IR (KBr, ¯max/cm¹1):
2943, 2865 (CH2, CH3), 1716 (C=O in Ar-COO-), 1600, 1510
(C-C in Ar), 1253, 1167 (COC), 1634 (C=C), 981 (=C-H).
1H NMR: (CDCl3, ¤): 0.69 (m, 3H, CH3), 0.85-1.04 (m,
12H, CH3), 1.10-2.40 (m, 36H, CH2), 3.97-4.19 (t, 4H, OCH2),
4.74-4.75 (m, 1H, OCHCH2 in Chol.), 5.40 (t, 1H, CHCH2 in
Chol.), 5.80-5.83 (dd, 1H, CH2=CH), 6.08-6.15 (dd, 1H,
CH2=CH), 6.26-6.30 (dd, 1H, CH=CH), 6.37-6.38 (dd, 1H,
CH2=CH), 6.86-6.89 (d, 2H, Ar-H), 7.26-7.44 (d, 2H, Ar-H),
7.60-7.64 (dd, 1H, CH=CH). Anal. Calcd for C45H66O5: C,
78.67; H, 9.68%. Found: C, 78.62; H, 9.66%.
Liquid crystals are partially ordered, anisotropic fluids that
are thermodynamically located between solid-state crystals with
three-dimensional order and isotropic liquids. Liquid crystalline
materials are generally divided into thermotropic and lyotropic
categories. The former mesophases, originating from their own
inherent molecular skeletons, are formed in response to a change
in temperature, while the latter mesophases form in the presence
of a suitable (isotropic) solvent. Some mesogens exhibiting both
thermotropic and lyotropic phases are called amphotropic.1
Lyotropic liquid crystals can be classified into three sub-
groups on the basis of the nature of component molecules:
amphiphilic, chromonic, and polymeric lyotropic liquid crys-
tals.2 Unlike amphiphilic compounds such as ionic and nonionic
surfactants, which consist of a hydrophilic head and a lipophilic
tail, chromonic lyotropic liquid crystals are formed in highly
concentrated aqueous solutions of water-soluble, disk-shaped,
polyaromatic molecules with hydrophilic substituents. Polymers
with a relatively rigid backbone in a planar or helical
conformation and with relatively flexible side groups attached
to the main chains show the greatest potential to develop
lyotropic liquid crystal phases.3
In recent years, interest in cholesteric liquid crystals has
increased considerably as a result of their unique optical
properties, namely, large optical rotation and selective light
reflection, which are caused by the helical structure of the
cholesteric mesophase. The selective reflection wavelength (-),
based on the pitch length of the helical structure for normal
incident light, is given by - = np, where n indicates the average
refractive index, and p is the helical pitch length. The selective
reflection wavelength can be altered by adjusting the helical
pitch length under the action of various external factors such as
temperature, electrical fields, or mechanical stress. Hence,
cholesteric liquid crystalline materials have found many appli-
cations in twisted nematic displays, thermography, linear and
nonlinear optics, sensors, and in novel electro- and magnetooptic
devices and detectors.4
Phase transition temperature and liquid crystal phases of
chiral monomer M1 were identified using differential scanning
O
H
O
The cholesteric mesophase is usually exhibited by chiral
calamitic molecules, mixtures of calamitic compounds with
chiral dopants, and cholesteryl derivatives that also form
thermotropic liquid crystals. However, lyotropic cholesteric
systems are less common and are based on cellulose derivatives,
O
O
O
H
H
Scheme 1. Synthesis of chiral monomer M1.
Chem. Lett. 2010, 39, 485-487
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

486
(a)
(b)
(a)
(b)
(c)
5
10
15
20
25
30
5
10
15
20
25
30
2θ/°
2θ/°
Figure 1. (a) Isotropic phase (M1/CH2Cl2 = 30/70), (b)
plannar texture (M1/CH2Cl2 = 60/40), and (c) crystal phase
of pure M1; POM textures were measured at room temperature.
Figure 2. XRD patterns for (a) typical lyotropic liquid crystal
state (M1/CH2Cl2 = 60/40) and (b) spherulites state (pure M1).
calorimetry (DSC) and polarized optical microscopy (POM),
respectively. Results of POM analysis show the M1 film displays
a Grandjean texture under POM at 80 °C. This result suggests
that M1 is a cholesteric liquid crystal. The colorful appearance of
the M1 film at 80 °C is due to the selective reflection of the
incident light. Furthermore, M1 was found to dissolve in both
dichloromethane (CH2Cl2) and chloroform (CHCl3). The optical
properties of selective light reflection caused by the variation of
solvent content were studied. Lyotropic liquid crystal phases of
the mixtures of M1 and dichloromethane with various weight
ratios were confirmed by POM. As seen in Figure 1, isotropic,
lyotropic, and crystalline phases of the M1/CH2Cl2 mixture
were confirmed. Figures 1a, 1b, and 1c show isotropic phase,
typical planar texture of cholesteric liquid crystals, and spher-
ulites of crystals, respectively. These results suggest self-
assembly of M1 when dissolved in dichloromethane solvents
at the proper weight ratio. A lyotropic phase was found to exist
for M1/CH2Cl2 weight ratios between 55/45 and 70/30. Similar
phenomena could be seen in the case of M1/CHCl3 mixtures,
although the exact weight ratios of M1/CHCl3 were not
investigated. In the case of the lyotropic liquid crystal phase
formed with CH2Cl2, the characteristic reflection color revealed
a blue shift from green to purple as the ratio of solvent to
monomer was gradually decreased. These results suggest that
reducing solvent ratios may change the arrangement of lyotropic
liquid crystals and shorten the helical pitch, resulting in the
observed blue shift. High ratios of CH2Cl2 to M1 may lead to the
formation of an isotropic liquid phase.
From the calculated molar ratio of the lyotropic system, one
M1 molecule is arranged in an ordered unit with 3-6 CH2Cl2
molecules. X-ray diffraction analysis (XRD) was used to
investigate the molecular arrangement of these lyotropic liquid
crystals. Figure 2a shows the representative XRD pattern for a
lyotropic liquid crystal state with a weak board amorphous peak
centered at about 2ª = 17.5°, corresponding to a lateral packing
distance between mesogenic groups of approximately 5 ¡. These
results suggest the presence of two CH2Cl2 molecules between
two Ml molecules, if we assume that the size of CH2Cl2
corresponds to the calculated value of 2.5 ¡. As compared with
the calculated lyotropic molar ratio, some solvent molecules
may exist around the highly ordered M1/CH2Cl2 cluster,
helping to preserve the helical structure of the cholesteric liquid
crystal phase. In contrast, for pure M1 in a crystalline state, the
appearance of sharp peaks in the low angle region corresponding
to spherulites was observed from XRD patterns as shown in
Figure 2b.
benzene rings and C=C bonds is expected as well. Finally,
chiral effects from cholesteryl groups may exhibit some steric
interaction. The effect of these intermolecular forces acting in
concert is the formation of a thermotropic, cholesteric liquid
crystal phase. It has been observed that, for small molecular
systems, hydrogen bonding, hydrophilic-hydrophobic interac-
tions, and ionic interactions could play important roles in the
molecular assembly process required to form lyotropic liquid
crystals.2 In this study, the solvent effect on the development of a
lyotropic liquid crystal phase may have been due to interactions
of solvent molecules around the arranged M1/CH2Cl2 cluster
through van der Waals or dipole-dipole interactions, forces that
are partially analogous to the molecular interactions of thermo-
tropic liquid crystals. The arrangement of suitable solvent
molecules surrounding the M1 molecules might supply fluidity
and maintain the necessary birefringence to obtain a lyotropic
liquid crystal phase.
Temperature variation and UV irradiation of the lyotropic
liquid crystal cell were also performed in order to investigate the
effects of temperature and light on the helical pitches within the
liquid crystal. Figure 3a shows the temperature dependence of
selective light reflection for a sealed lyotropic liquid crystal cell
containing a 60/40 (w/w) mixture of M1/CH2Cl2. As the
temperature increased from 30 to 55 °C, the reflected light
displayed a blue shift from a peak wavelength of approximately
575 to 470 nm. It was noteworthy that the temperature-induced
variation was in the visible region and was completely
reversible. These results suggest that temperature variations
could change the arrangement of assembled molecules, thereby
tuning the pitch of the helical structure. Theoretically, the central
wavelength and reflection band of the reflection light could be
calculated using following equations:
-0 ¼ np
ꢀ- ¼ ꢀnp
ð1Þ
ð2Þ
where n indicates the average refractive index, -0 is the central
wavelength of the reflected light, ¦n is the birefringence of the
liquid crystal, and p is the helical pitch length. The pitch of the
helical structure and molecular order within the liquid crystal
may also impact the optical properties of reflected light.
The synthesized compound M1 contains a cinnamoyl group
and reveals E-Z photoisomerization during UV irradiation,
resulting in conformational changes from the linear E form
to the bent Z form. The effects of UV exposure on selective
light reflection from lyotropic cholesteric liquid crystals con-
taining M1 were also investigated. As shown in Figure 3b, an
irreversible blue shift of selective reflection bands was observed
for UV irradiation of a sealed cell containing a 60/40 (w/w)
As seen in Scheme 1, the carbonyl groups and oxygen
atoms in M1 may exhibit polarity and display a certain level of
molecular interaction. In addition, ³-³ interaction between
Chem. Lett. 2010, 39, 485-487
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

487
120
100
80
60
40
20
0
(a)
and in particular to the linkage of a photoisomerizable
cinnamoyl group to a cholesteryl group, resulting in mesogenic
and chiral properties that affect not only the helical twisting
power of the chiral group but also the configuration of
mesogenic fragments in the lyotropic liquid crystal phase.
Photo-induced E-Z isomerization should also affect the polarity
of M1. In addition, the solvent molecules surrounding M1 may
also stabilize the molecular orientation of M1 to maintain helical
structure during UV irradiation. Due to the Bragg reflection, a
significant reflected color could be seen.
55°C
50°C
45°C
40°C
35°C
30°C
400
500
600
700
Wavelength/nm
In conclusion, we have synthesized a novel functional
monomer, M1, end-capped with a cholesteryl group. The
monomer reveals amphotropic properties. Thermochromic prop-
erties of M1 and photochromic properties of a lyotropic liquid
crystal of M1/CH2Cl2 with a 60/40 weight ratio were inves-
tigated. Increasing cell temperature caused a reversible blue
shift, while brief UV exposure caused an irreversible E-Z
configurational isomerization. However, longer periods of UV
irradiation caused photodegradation.
120
(b)
100
80
60
40
20
0
25 min
20 min
15 min
10 min
5 min
0 min
400
500
600
700
Wavelength/nm
Recerences
Figure 3. Dependence of (a) temperature and (b) UV irradi-
ation on selective light reflection for a sealed lyotropic liquid
crystal cell containing a 60/40 (w/w) mixture of M1/CH2Cl2.
Reflection light of the cells is located in visible reason.
1
2
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1
mixture of M1/CH2Cl2 at 40 °C. According to H NMR analy-
sis, the blue shift of selective reflection bands of the lyotropic
cholesteric liquid crystal following UV irradiation is attributed
to the irreversible photoisomerization of the C=C bond in the
M1 monomer’s cinnamoyl functional group. It is noteworthy
that more than 1 h of UV exposure may destroy the helical
structure entirely, resulting in decreased transmittance of wave-
lengths in the selective reflection band (results not shown).
Overall, these results suggest that brief (<1 h) UV exposure may
produce irreversible E-Z isomerization. However, longer peri-
ods of UV irradiation may cause serious degradation of the M1
molecule.
Nevertheless, it is interesting to observe that many other
studies of chiral photochromic compounds in liquid crystal
matrices have reported changes in the configuration of the chiral
molecular fragments in the course of the isomerization process.
These changes are often accompanied by a decrease in the
anisometry of chiral constituents and, as a consequence, their
helical twisting power, resulting in unwinding of the helical
structure and a red shift of the selective reflection band.7
As seen in Figure 3b, the unusual phenomenon of blue
shifts in this study is ascribed to the molecular structure of M1,
3
4
5
6
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7
Chem. Lett. 2010, 39, 485-487
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/