Novel asymmetrical single- and double-chiral liquid crystal diads with wide blue phase ranges

Article abstract of DOI:10.1039/c4ra13943g

In this study, two series of novel asymmetrical single- and double-chiral liquid crystal diads using a central linker to link two different mesogenic cores were successfully synthesized. The effects of the position of chiral centers and the number of aromatic rings on the mesomorphic and electro-optical properties were investigated. We found that diads III-N (where N = A, B, C and D) have better mesophasic properties than II-N (where N = A, B, C, D and E). Compared with III-C with a chiral center at the terminal alkoxyl chain, diad III-D exhibited the widest temperature range of BPI (ca. 31°C) as the chiral center was introduced to the central linker. Besides III-D, BPs were also observed in diads II-B and III-B with a chiral center at the central spacer. According to our experimental results and molecular modeling, the mesomorphic properties and temperature ranges of BPs will be affected by the values of biaxiality and dipole moment, along with the bent shape of molecular geometry. Therefore, we demonstrated the first example of asymmetrical single- and double-chiral liquid crystal diads involved chiral centers located at the central linker to exhibit BPs, including the mesophases of BPI and BPIII, which might offer new BP single-components to the future applications of eutectic liquid crystal mixtures with wide BP ranges.

Full text of DOI:10.1039/c4ra13943g

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Novel asymmetrical single- and double-chiral
liquid crystal diads with wide blue phase ranges†
Cite this: RSC Adv., 2015, 5, 4615
Chong-Lun Wei,a Te-Cheng Chen,a Putikam Raghunath,b Ming-Chang Linb
a
*
and Hong-Cheu Lin
In this study, two series of novel asymmetrical single- and double-chiral liquid crystal diads using a central
linker to link two different mesogenic cores were successfully synthesized. The effects of the position of
chiral centers and the number of aromatic rings on the mesomorphic and electro-optical properties
were investigated. We found that diads III-N (where N ¼ A, B, C and D) have better mesophasic
properties than II-N (where N ¼ A, B, C, D and E). Compared with III-C with a chiral center at the
terminal alkoxyl chain, diad III-D exhibited the widest temperature range of BPI (ca. 31 ꢀC) as the chiral
center was introduced to the central linker. Besides III-D, BPs were also observed in diads II-B and III-B
with a chiral center at the central spacer. According to our experimental results and molecular modeling,
the mesomorphic properties and temperature ranges of BPs will be affected by the values of biaxiality
and dipole moment, along with the bent shape of molecular geometry. Therefore, we demonstrated the
first example of asymmetrical single- and double-chiral liquid crystal diads involved chiral centers
located at the central linker to exhibit BPs, including the mesophases of BPI and BPIII, which might offer
new BP single-components to the future applications of eutectic liquid crystal mixtures with wide BP
ranges.
Received 6th November 2014
Accepted 5th December 2014
DOI: 10.1039/c4ra13943g
www.rsc.org/advances
very narrow ranges of BPLCs were observed, polymer stabilized
BPLCs was rst discovered by Kikuchi et al. to extend the
temperature range of BPs phase over 60 ꢀC.13 Since BPLC
arrangements were stabilized by the polymer network design to
enlarge the BPLC ranges, general interests have been attracted
to develop BPLCs recently. Some advantages in the elds of
optoelectronics and photonics are listed as follows:14,15 fast
response time (about sub-millisecond), no alignment layer
required,16 and free of typical birefringence.17
Introduction
It is well known that liquid crystal molecules may reveal many
different mesophases, including nematic,1 smectic,2 chole-
steric,3 and blue phases, and these are attributed to their
chemical structures, such as molecular congurations with
specic shapes and dipole moments. Among these mesophases,
blue phase liquid crystals (BPLCs) are observed between
isotropic phase and chiral nematic (N*) phase during cooling
process, which have less applications due to their narrow
mesophasic ranges (usually about 1 K).4 It has been reported
that BPLCs aggregate into internal helical alignments called
“double twist cylinders (DTC)”,5,6 where are three types of
BPLCs: blue phase I (BPI), blue phase II (BPII) and blue phase III
(BPIII). The packing structures of these three BPLCs are
described as follows: BP I is a body-center cubic structure, BP II
is a simple cubic structure,7,8 and BP III is the same symmetry as
the isotropic phase with arbitrary orientation.9,10 BPLCs have
different performances in the eld of optoelectronics: among all
BPs, BP III has the fastest response speed,11 and BP II shows
smaller hysteresis and faster response time than BP I.12 Though
Nowadays, several research groups have reported numerous
methods to induce the blue phase and enlarge the temperature
range of blue phase, which were accomplished either by struc-
tural modications or mixture optimizations. For example,
Yang et al. utilized hydrogen bonds to stabilized BPLCs, where
two complimentary hydrogen-bonded moieties were self-
assembled by hydrogen bonds to form a BP complex with a
wide temperature range about 23 ꢀC.18 In addition, they also
tried to dope hydrogen-bonded bent-shaped and T-shaped
molecules to stabilize BPs.19 Pivnenko et al. synthesized
bimesogens structures to induce BPLCs with a BP temperature
range ca. 44 C.20 Besides, U-shaped molecules have also been
ꢀ
doped with a chiral additive to extend the BP range success-
fully.21 There are some reports demonstrated that several biaxial
nametic LCs doped with certain amounts of chiral materials can
enhance the temperature range of BPs.22–24 Although BPLCs
have so many benets, very limited single-component BPLC
molecules with wide BPLC ranges have been developed so far.
aDepartment of Materials Science and Engineering, National Chiao Tung University,
Hsinchu, Taiwan, Republic of China. E-mail: linhc@mail.nctu.edu.tw
bCenter for Interdisciplinary Molecular Science, Department of Applied Chemistry,
National Chiao Tung University, Hsinchu, Taiwan, Republic of China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra13943g
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Lately, many experts have designed special molecular structures obtained using the Gaussian 09 ab initio/DFT quantum chem-
with different shapes to exhibit or stabilize BPLC mesophases: ical package.39
rod-like,25 bent-core,26 T-shape,27 discotic28 and (asymmetrical
or symmetrical) diad29 architectures. Yoshizawa et al. used a
Liquid-crystalline and physical properties
binaphthyl structure to stabilize the blue phase with a
The phase transition behaviors of all nal asymmetrical diads
were characterized by polarizing optical microscopy (POM)
using a Leica DMLP equipped with a temperature control hot
temperature range of 30 ꢀC approximately,30 interestingly,
various structures of liquid crystal diads have been reported,31–34
but the mesophasic behavior of liquid crystals almost only
stage (Mettler Toledo FP82HT). Temperatures and enthalpies of
showed the nematic or smectic phases. However, Takezoe et al.
phase transitions were determined by differential scanning
linked a exible spacer between a rod and a cholesterol meso-
calorimetry (DSC, model: Perkin Elmer Pyris 7) under N2 at a
genic unit which exhibited a wide range of BPs.35 Our previous
heating and cooling rate of 1 ꢀC minꢁ1. The performance of
research also reported a bent-core oxadiazole-based liquid
optical properties was determined in the range of blue phase
crystal to possess a blue phase with a wide range of ca. 30 ꢀC.36
temperature and the optical transmittance was studied with a
function of applied electric AC eld at 2 kHz. The transmittance
In this study, we synthesized two series of novel asymmet-
rical single- and double-chiral liquid crystal diads with an odd-
with 100% and 0% were calibrated by that the cell under
number spacer to link two different mesogenic rods and
parallel polarizers. The distance between electrodes was 10 mm
introduced chiral centers into the central linker and/or exible
and the cell gap was maintained at 7.5 mm by spacers.
terminus of the diads. We demonstrated the effects of the
number of aromatic rings and the positions of the chiral centers
on the mesophasic behaviors of LCs. Interestingly, we found
one of the double-chiral liquid crystal diad exhibited BPI with a
wide temperature range about 31 ꢀC. Moreover, the phase
transition sequences, electric-optical properties, Kerr constants
and theoretical calculations of molecular modeling were
investigated in this report.
Preparation of materials
(i) (S)-6-((40-Cyano-[1,10-biphenyl]-4-yl)oxy)-4-methylhexyl 4-
((4-((4-((R)-octan-2-yloxy)benzoyl)oxy)benzoyl)oxy)benzoate, (III-
D). The synthesis of compound III-D was shown in Scheme 1. To
a stirred solution of M1 (3.0 g, 9.7 mmol), M2 (2.42 g, 4.9 mmol)
and 4-(N,N-dimethylamino)pyridine (DMAP) (0.13 g, 1.06 mmol)
in dry dichloromethane (100 mL), N,N-dicyclohex-
ylcarbodiimide (DCC) (2.35 g, 11.6 mmol) was added and the
reaction mixture stirred at room temperature overnight under
nitrogen. The resulting precipitate of dicyclohexylurea (DCU)
was ltered off and washed with an excess of dichloromethane
(20 mL). The solvent was evaporated and the crude product was
Experimental
Spectroscopic analysis
1H NMR spectra were recorded on a Bruker Unity 300 MHz
puried
by
silica
gel
chromatography
(n-hexane/
spectrometer using DMSO-d6, CDCl3 and THF-d8 as solvents.
Elemental analyses (EA) were performed on a Heraeus CHN-OS
RAPID elemental analyzer. Synchrotron powder X-ray diffrac-
tion (XRD) measurements were performed at beam-line BL17A
equipped with magnetic of the National Synchrotron Radiation
Research Center (NSRRC), Taiwan, where the wavelength of
dichloromethane ¼ 1 : 1 v/v) and then recrystallized from
methanol/dichloromethane (5 : 1 v/v) to afford compound III-D
as a white solid in 78% yield. 1H NMR (300 MHz, CDCl3):
d (ppm) 8.27 (d, J ¼ 8.7 Hz, 2H), 8.17 (d, J ¼ 8.7 Hz, 2H), 8.14
(d, J ¼ 9.0 Hz, 2H), 7.71–7.63 (m, 4H), 7.54 (d, J ¼ 8.7 Hz, 2H),
7.40 (d, J ¼ 8.7 Hz, 2H), 7.31 (d, J ¼ 9.0 Hz, 2H), 7.02–6.98
(m, 4H), 4.53–4.51 (m, 1H), 4.37 (t, J ¼ 6.3 Hz, 2H), 4.09 (m, 2H),
1.92–1.84 (m, 5H), 1.52–1.33 (m, 15H), 1.35 (d, J ¼ 6.6 Hz, 3H),
0.91 (t, J ¼ 6.6 Hz, 3H). Anal. calcd for C49H51NO8: C, 75.27, H,
6.57, N, 1,79; found: C, 75.31, H, 6.62, N, 1.83%.
˚
X-ray was 1.33336 A. The powder samples were packed in a
capillary tube and heated by a heat gun, for which the
temperature controller is programmable by a PC with a PID
feedback system. The scattering angle theta was calibrated by a
mixture of silver behenate and silicon.
Results and discussion
Chemical synthesis
Molecular simulation method
The gas phase optimization of all asymmetrical structures of
liquid crystal diads II-N (N ¼ A, B, C, D and E) and III-N (where N
¼ A, B, C and D) was carried out with the generalized gradient
approximation (GGA) density functional B97D along with
dispersion correction37 using the standard 6-31G(d,p) basis set.
The predicted minima were further conrmed by frequency
calculations at the same level. Only the lowest energy confor-
mations are reported here. The electrostatic potential derived
charges (ESP) were calculated according to Merz–Singh–Koll-
man (MK) scheme38 at the B97D/6-31G(d,p) level of theory. ESP
As illustrated in Fig. 1, series II-N (where N ¼ A, B, C, D and E)
and III-N (where N ¼ A, B, C and D) were synthesized according
to Schemes S1–S4 (see the ESI†), where one of the mesogenic
rods in series III are longer than series II, along with B, C and D
represent single- and double-chiral diads, respectively. These
1
compounds with high purities were fully characterized by H-
and 13C-NMR, as well as mass ESI spectra (see the ESi†.)
Mesophasic and thermal properties
results were used to compute maps of electrostatic potential. As shown in Table 1, the phase transition temperatures,
The results of all the calculations reported in this work were enthalpies and mesophasic ranges of the asymmetrical liquid
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Scheme 1 Synthesis of III-D.
crystal diads were determined by differential scanning calo- power but didn't diminish the interactions of mesogenic cores.
rimetry (DSC) and polarizer optical microscope (POM). The However, as the chiral center was shied to the terminal alkoxyl
transition temperature of BPs–chiral nematic (N*) was deter- chain, the mesophases were totally vanished in II-C due to a
mined by POM (at a cooling rate of 0.5 C minꢁ1) due to their stronger chiral twisting of the chiral center closer to the rigid
ꢀ
undetectable enthalpy changes by DSC. To realize the effect of core. Moreover, we also introduced double chiral centers to
various asymmetrical congurations, including different both central linker and exible terminus of diad II-D, which do
numbers (or positions) of chiral centers and aromatic rings, on not have any mesophases due to even a stronger chiral twisting
mesophasic and thermal properties of these asymmetrical than II-C to further eliminate mesogenic interactions of meso-
single- and double-chiral liquid crystal diads, the comparison of phases. The strongest chiral twisting of both diads II-C and II-D
all compounds are described as follows:
to reduce the mesogenic interactions of rigid cores were also
(i) Compounds II-N (where N ¼ A, B, C, D and E). evidenced by their lowest crystallization (as well as isotropiza-
Compound II-A (no chiral center) exhibits a phase transition tion) temperatures among all diads. Among these described
sequence of iso–nematic (N)–smectic A (SmA)–crystal. We tried chiral diads (II-B, II-C and II-D), only diad II-B with a single
to introduce a single chiral center to the central linker and chiral center at the central spacer displayed mesophases,
exible terminus of diads II-B and II-C, respectively. In contrast including BPIII. Hence, another single chiral diad II-E was
to II-C, diad II-B revealed BPIII (ranging 3.4 ꢀC) due to the chiral introduced to investigate the effects of chiral center position at
center located at the central spacer, which induced twisting the central spacer, which showed a phase transition sequence of
iso–N*–crystal. Diad II-E with a chiral center at the central
spacer possessed a mesophase of N*, but BPIII was absence in
II-E due to the different location of chiral center in contrast to
II-B. Accordingly, a single chiral center located at the appro-
priate position of the central linker was required for diad II-B to
obtain BPIII.
(ii) Compounds III-N (where N ¼ A, B, C and D). The phase
transition sequence of compound III-A is similar to II-A, i.e.,
iso–N–SmA–crystal, but III-A has higher transition temperatures
and wider mesophasic ranges (i.e., N and SmA) than II-A due to
the longer rigid core of III-A with a stronger p–p interaction.
Similar to II-B and II-C, a single chiral center was introduced to
the central spacer and exible terminus of diads III-B and III-C,
respectively. In comparison with III-C, single-chiral diad III-B
exhibited BPI (ranging 1.5 ꢀC) due to the chiral center located at
the central spacer. Nevertheless, as the chiral center was shied
to the terminal alkoxyl chain, III-C had similar mesophases of
III-B, i.e., N* and SmA, but BPI was totally disappeared in III-C
due to a stronger chiral twisting of the chiral center closer to the
Fig. 1 Chemical structures of compounds II-N (where N ¼ A, B, C, D
and E) and III-N (where N ¼ A, B, C and D).
rigid core. Because of the same reason, compared with III-B,
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Table 1 Phase transition temperatures (ꢀC) and enthalpies (J gꢁ1) of asymmetrical liquid crystal diadsa,b
Phase transition temperature (ꢀC)
Compound
II-A
[enthalpies (J gꢁ1)]
DTBP (ꢀC)
DTN* (ꢀC)
DTN (ꢀC)
DTSmA (ꢀC)
32.1
Iso 99.2 [0.41] N 68.7 [1.89] SmA 36.6
[2.03] Cr
30.5
II-B
Iso 93.8 [0.54] BPIII 90.4c N* 70.4 [2.31]
SmA 44.3 [2.11] Cr
3.4
20.0
26.1
II-C
II-D
II-E
III-A
Iso 23.8 [3.57] Cr
Iso 24.9 [16.11] Cr
Iso 41.8 [0.10] N* 30.9 [0.83] Cr
Iso 195.0 [1.12] N 119.1 [2.99] SmA 63.0
[1.51] Cr
10.9
75.9
56.1
33.7
7.9
III-B
III-C
Iso 182.2 [0.94] BPI 180.7c N* 106.2 [2.44]
SmA 72.5 [1.22] Cr
1.5
74.5
21.9
24.7
Iso 109.5 [11.76] N* 87.6 [5.43] SmA 79.7
[2.03] Cr
III-D
Iso 108.0 [0.36] BPI 76.6c N* 51.9 [1.63] Cr
31.4
a
b
Peak temperatures in the DSC proles obtained during the rst heating and cooling cycles at a rate of 1 ꢀC minꢁ1
.
Iso ¼ isotropic phase; BPIII ¼
blue phase III; BPI ¼ blue phase I; N ¼ nematic phase; N* ¼ chiral nematic phase; SmA ¼ smectic A phase; Cr ¼ crystal. c The transition to this
phase was observed under the polarizing optical microscope and it was too weak to be recognized by the DSC. (Phase transition temperatures and
enthalpies of heating process are shown in Table S1.)
most transition temperatures and both mesophasic ranges of the POM texture of III-B at 181.0 ꢀC upon cooling (0.5 ꢀC minꢁ1
)
N* and SmA were reduced in III-C. Accordingly, SmA was totally is also illustrated in Fig. S1.† Meanwhile, the phase transitions
removed and BPI (ranging 31 ꢀC) was induced in double-chiral of iso–BPI–N* in III-D are shown in Fig. 3(a)–(c), respectively. All
diad III-D owing to the introduction of double chiral centers to phase transitions were investigated by XRD measurements and
both central spacer and exible terminus. Similarly, the stron- their related data are shown in Fig. S2† and Table S2† of the ESI.
gest chiral twisting of diad III-D to decrease the mesogenic Generally, sharp reection peaks of XRD patterns (representing
interactions of rigid cores was also evidenced by its lowest phase layer d-spacings) were observed in the small angle area of the
transition temperatures among all diads in series III.
smectic phase (including SmA), but only broad peaks were
(iii) Comparison of series II and III. All phase transition revealed in the wide angle area of the N and N* phases. As
temperatures of the corresponding phases in compounds III-N shown in Table S2† (see the ESI), II-A and II-B processed d-
˚
˚
were higher than those of compounds II-N (where N ¼ A, B, C, D spacing values of 41.8 A and 43.4 A, respectively. In addition, III-A,
˚
˚
and E) because of the longer rigid core of series III-N (where N ¼ III-B and III-C also exhibited d-spacing values of 49.7 A, 50.1 A and
˚
A, B, C and D) with stronger p–p interactions. Among all diads, 45.7 A, respectively. Their d-spacing values (d) were a little larger
only diads II-C and II-D did not have any mesophases owing to than their corresponding molecular lengths (L) by the theoretical
their strongest chiral twisting with less mesogenic interactions simulation (shown below), which suggested the d-spacing values
of shorter rigid cores. In contrast to II-C and II-D, compounds of the SmA phase in these diads are interdigitated layer
III-C and III-D with a longer rigid core possessed stronger p–p arrangements (i.e., L < d < 2L). Therefore, the SmA phase was
interactions to compensate the chiral twisting thus to preserve veried not only by the focal-conic fan texture of POM photo-
the molecular assembly of mesophases. Interestingly, among all images but also by the d-spacing values of sharp reection peaks.
asymmetrical diads, BPs only exited in single-chiral diads II-B,
The II-B behavior of BPIII could be veried by the rotation of
III-B and double-chiral diad III-D, which all possessed chiral the polarizer (see Fig. 4). As the polarizer was rotated clockwise
centers at central spacer. Compared with BPIII of II-B, the other
diads III-B and III-D possessed BPI owing to their longer rigid
core. Above all, double-chiral diad III-D was discovered to
possess the widest BP temperature range among all diads,
which is the rst molecular design of liquid crystal diad with
double-chiral centers (at both central spacer and exible
terminus) in the literature.
Optical and XRD investigations
Regarding the blue phases of diads II-B, III-B and III-D, we
Fig. 2 POM textures of II-B upon cooling (0.5 ꢀC minꢁ1). (a) Isotropic
investigated the phase transitions by POM on cooling process
state at 100 ꢀC. (b) Blue phase III at 92.1 ꢀC. (c) Chiral nematic phase at
80.2 ꢀC. (d) Smectic A phase at 60.1 ꢀC. (Scale bar: 40 mm. White
(cooling rate 0.5 C minꢁ1). Fig. 2(a)–(d) show the phase tran-
ꢀ
sitions of iso–BPIII–N*–SmA in II-B, respectively. In addition, arrows are the directions of polarizers and analyzers.)
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Fig. 3 POM textures of III-D upon cooling (0.5 ꢀC minꢁ1). (a) Isotropic
state at 112.3 ꢀC. (b) Blue phase I (platelet textures with different colors
and stripes) at 86.2 ꢀC. (c) Chiral nematic phase at 65.1 ꢀC. (Scale bar:
40 mm. White arrows are the directions of polarizers and analyzers.)
Fig. 5 Optical transmittance of diads II-B and III-D in BPs (at T ꢁ
TI-BP ¼ ꢁ2 ꢀC).
by a small angle of 10ꢀ from the crossed position, the color was
changed from blue to light blue (Fig. 4(a)). Upon rotating the
polarizer counterclockwise by the same angle (10ꢀ) from the
crossed position, the color was changed from blue to red
(Fig. 4(c)). The POM observations also indicated that the phase
of BPIII has selective reection colors according to their pitch
lengths, which were examined by changing the cross angles in
the polarizer and analyzer of POM.
hysteresis is DV/Von (DV is the voltage difference between the
forward and backward directions at half of the peak transmittance
and Von is the driving voltage).41 The electro-optical switching
exhibited that II-B did not have any hysteresis (ꢂ0%), but III-D
was observed to have a large hysteresis (ꢂ27%). Compared with II-
B, the higher applied voltages (10% and 90% transmittance in the
V–T curve) and hysteresis of III-D were due to a larger number of
aromatic rings in III-D with stronger p–p interactions.
The values of response time for diads II-B and III-D were
obtained from the electro-optical response curves of Fig. S3(a)
and (b),† respectively (see the ESI). As summarized in Fig. 6,
their values of response time were measured with a rectangular
wave function of the AC eld (at a frequency of 2 kHz) for the
rise and decay processes at T ꢁ TI-BP ¼ ꢁ2 ꢀC. The denitions of
the rise time (son) and decay time (soff) are the response time
values for 90% transmittance change by increasing and
Electro-optical studies
In order to investigate the electro-optical switching behavior in
the blue phase of diads II-B and III-D, a rectangular wave
function of the AC eld at a frequency of 2 kHz was applied to
measure the optical transmittance. Fig. 5 shows the voltage–
transmittance curve at T ꢁ TI-BP ¼ ꢁ2 ꢀC, where the black lines
with solid squares and circle squares represent the trans-
mittance of compound II-B upon increasing and decreasing
voltages, respectively. The red lines with solid dots and circle
dots stand for the transmittance of III-D upon increasing and
decreasing voltages, respectively. Both II-B and III-D showed
enhanced transmittance by increasing the electric eld. As
transmittance reached 10% and 90% in the V–T curve, we found
that III-D had higher respective electrical elds (11.5 and 17.2 V
mmꢁ1) than II-B (10.2 and 15.6 V mmꢁ1). In addition, in-plane-
switching (IPS) cells were utilized to realize the V–T hysteresis
by increasing the voltage to saturation. Because hysteresis will
inuence the grayscale of liquid crystal device, the hysteresis
should be minimized for real applications.40 The denition of
Fig. 4 Optical images of II-B observed by POM at 92.7 ꢀC (a) polarizer
was rotated clockwise by a small angle of 10ꢀ. (b) Polarizer and Fig. 6 Diads II-B and III-D in the BP at T ꢁ TI-BP ¼ ꢁ2 ꢀC. (a) Electric-
analyzer were orthogonal. (c) Polarizer was rotated counterclockwise field dependence of the response times for the rise processes. (b)
by a small angle of 10ꢀ. (White arrows are the directions of polarizers Electric-field dependence of the response times for the decay
and analyzers.)
processes. (c) Electric-field dependence of the total response times.
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decreasing voltage, respectively.42,43 Both son values of II-B and
The front and side views of efficient molecular structures, III-
III-D in Fig. 6(a) were found to be reduced concomitantly by D, III-B and II-B are shown in Fig. 7 and remaining other
increasing voltage, but their soff values in Fig. 6(b) are enhanced geometries are shown in the ESI Fig. S4.† The biaxiality, bent
by increasing voltage. The behaviour of increasing soff values angle and dipole moment of molecular geometry play a very
with higher voltages can be explained by the unwinding helix important role in widening and stabilizing the BP temperature
mode of BPLCs stabilized by a higher voltage which is more range.48 The biaxiality is dened as the ratio of two distin-
difficult to relax back easily via the removal of electric elds.44 In guishable short axes (W1, W2) of the molecule with a at shape
the meanwhile, we found that the rise time son of III-D is smaller (i.e., possessing a more non-circular cross section), which is
than II-B, but the decay time soff of III-D is larger than II-B. Thus, different from the uniaxiality with two identical short axes (W).
the values of rise time (son) and decay time (soff) have opposite Hence, a similar molecular structure with a more bent shape
trends for diads II-B and III-D. As a result, the values of total (i.e., a smaller bent angle in Fig. 7) should have a larger biaxiality
response time (stotal ¼ son + soff) in Fig. 6(c) are enhanced by (i.e., a larger difference between W1 and W2). These parameters
increasing voltage, which has the same trend as the decay time are dened as following: L (length along the long axis), biaxiality
(soff) due to the larger values of soff with more contributions to equals W1/W2 (W1: width along the short axis normal to the
stotal compared with those of son. According to Table S3,† the benzene plane and W2: width along the short axis parallel to the
average values of total response time (stotal ¼ son + soff) for II-B benzene plane) and bent angles are measured as the angles
and III-D are 2.74 and 3.56 ms, respectively, where II-B has the between the centers of the rst, central and nal benzene rings
fastest total response time.
of the bent-core structures (see Fig. 7 and Table 2).
According to the denition of the Kerr effect,45,46 the formula:
Dninduced ¼ lKE2, where Dn is the electric eld-induced bire-
fringence, l is the wavelength of light, K is the Kerr constant and
E is the electric-eld. The Kerr constants of II-B and III-D in
their corresponding BPs were 2.2 ꢃ 10ꢁ11 and 2.7 ꢃ 10ꢁ11
mVꢁ2, respectively, where BPI of III-D demonstrated the largest
Kerr constant. The BPs with large eld-induced birefringences
would cause the orientation changes of the ordered molecules
easily. In contrast to nitrobenzene with a well-known and large
Kerr constant 2.2 ꢃ 10ꢁ12 mVꢁ2
,
47 II-B and III-D possessed Kerr
constants 10 times larger than nitrobenzene might have useful
applications in the future.
Theoretical simulation
As alluded to above, we carried out geometry optimization at the
B97D/6-31G(d,p) level for all asymmetrical liquid crystal diads
II-N (N ¼ A, B, C and D) and III-N (where N ¼ A, B, C and D) to
analyze the structural variations. The main geometric parame-
ters for the ground states of lowest energy conformations of II-B,
III-B and III-D bearing chiral centers at the central linker and/or
terminal alkoxyl chain are summarized in Table 2 with molecule
length, breadth, bent angle and dipole moment, which can
inuence the temperature range and stabilization of blue pha-
ses (BPs). In addition, the related data of the other compounds
without BPs are shown in Tables S3–S5 (see ESI†).
Fig. 7 Molecular models of the lowest energy conformations for
single molecules of diads (a) III-D (b) III-B and (c) II-B.
Table 2 Calculated lengths, breadths, bent angles and dipole moments of molecular structures in optimized asymmetrical liquid crystal diads at
the B97D level with the 6-31G(d,p) basis set
Biaxial parameter
(W1/W2)
Dipole moment
(Debye, D)
Bent anglea (deg)
˚
˚
˚
Compounds
Length L (A)
Breadth W1 (A)
Breadth W2 (A)
II-B
III-B
III-D
39.5
45.0
43.2
8.8
10.8
12.1
6.0
6.4
6.5
1.47
1.69
1.86
139.4
141.1
132.1
10.9
13.1
13.2
a
Bent angle (ꢀ) measured as the angle between the rst, central and nal benzene rings' centers of the bent-core structures. The dipole moments of
the lowest energy structures are given (see ESI Tables S4–S6).
4620 | RSC Adv., 2015, 5, 4615–4622
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Comparing the BPꢀtemperature range of compound II-B and and the blue color refers to more positive charge and thus the
III-B, diad II-B (ꢂ3.4 C) has a little wider BP range than III-B green color signies the zero electrostatic potential. The highest
ꢀ
(ꢂ1.5 C), but diad III-B has one more aromatic ring than II-B. negative potential (red) is always located at the C(O)]O and
To explain this result, the difference of biaxiality, dipole C]N functionalities. In addition, the remaining electrostatic
moment and bent angle are further investigated between II-B potential maps of the other diads are also shown in the ESI
and III-B. Although the values of biaxiality and dipole moment Fig. S5.†
for diad III-B (biaxiality parameter ꢂ1.69 and dipole moment
ꢂ13.1 D) are larger than those of II-B (biaxiality parameter
Conclusions
ꢂ1.47 and dipole momentꢂ10.9 D), diad II-B (139.4ꢀ) has a
more bent shape than III-B (141.1ꢀ). In this study, we found that
the bent shape of molecular geometry is useful to stabilize the
Two series of novel asymmetrical chiral liquid crystal diads II-N
(where N ¼ A, B, C, D and E) and III-N (where N ¼ A, B, C and D)
were synthesized to possess two different mesogenic units
which were linked together by an odd-number spacer. The
effects of the position of chiral centers and the number of
aromatic rings on the mesophasic properties (especially for BPs)
were investigated. Among these diads, II-B exhibited blue phase
III (BPIII) and diads III-B and III-D revealed blue phase I (BPI),
where III-D showed the widest temperature of BPI (ꢂ31 ꢀC). The
effects of the number of aromatic rings and the position of
chiral centers on the behavior of mesophases, electric-optical
properties and Kerr constants were systemically investigated.
Based on the molecular modeling, we found that the large
values of biaxiality and dipole moment, along with the bent
shape of molecular geometry are useful to extend and stabilize
the temperature range of BPs. Consequently, the rst report of
asymmetrical single- and double-chiral liquid crystal diads
containing a chiral center at the central linker is illustrated to
exhibit BPs, including the mesophases of BPI and BPIII. The
single-component discoveries of novel liquid crystal diads with
wide BP ranges will pave a new way to optimize BP ranges in the
eutectic mixtures for future applications.
ꢀ
BPs so that the BP temperature range of diad II-B (ꢂ3.4 C) is
wider than III-B (ꢂ1.5 ꢀC). On the other hand, diad III-D has one
more chiral center at the terminal alkoxyl chain than III-B but
the BP temperature range of III-D (ꢂ31.4 ꢀC) is much wider than
that of III-B (ꢂ1.5 ꢀC). In order to explain this phenomenon, the
differences of biaxiality, dipole moment and bent angle were
studied between III-D and III-B. As shown in Table 2, diad III-D
(biaxiality parameter ꢂ1.86 and dipole moment ꢂ13.2 D) has
larger values of biaxiality and dipole moment than III-B (biax-
iality parameter ꢂ1.69 and dipole moment ꢂ13.1 D). Besides,
the molecular geometry of III-D (132.1ꢀ) is more bent than that
ꢀ
of III-B (141.1ꢀ) so that diad III-D (ꢂ31.4 C) with a more bent
shape (i.e., a larger biaxiality) is observed to have a wide
temperature range of BP. In the investigation of biaxiality, bent
angle and dipole moment effects on widening and stabilizing
the BP temperature range, the large values of biaxiality and
dipole moment, along with the bent shape of molecular
geometry are found to be useful to extend and stabilize the
temperature range of BP.
The electrostatic potential maps of all diads III-D, III-B and
II-B calculated at the B97D/6-31G(d,p) level are shown in Fig. 8.
The results reveal that the red color refers to the electron-rich
Acknowledgements
This project was nancially supported by the National Science
Council of Taiwan (ROC) through funding (no. NSC 101-2113-
M-009-013-MY2 and NSC 102-2221-E-009-174). We are grateful
to the National Center for High-performance Computing for
computer time and facilities. The powder XRD measurements
were supplied by beam-line BL17A (charged by Dr Jey-Jau Lee) of
the National Synchrotron Radiation Research Center (NSRRC)
in Taiwan.
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