A fluorinated binaphthyl chiral dopant for fluorinated liquid crystal blue phases

Article abstract of DOI:10.1039/c4tc01049c

A 6,6′-fluorinated binaphthyl enantiomer, (R)-1, was applied as a chiral dopant to produce cholesteric blue phases (BPs). A fluorinated nematic liquid crystal doped with (R)-1 showed lower critical chirality to induce a BP and a larger temperature range of BPs than that doped with a conventional chiral dopant. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4tc01049c

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A fluorinated binaphthyl chiral dopant for
fluorinated liquid crystal blue phases†
Cite this: J. Mater. Chem. C, 2014, 2,
6467
a
b
b
b
*
K. Kakisaka, H. Higuchi, Y. Okumura and H. Kikuchi
Received 20th May 2014
Accepted 18th June 2014
DOI: 10.1039/c4tc01049c
www.rsc.org/MaterialsC
A 6,60-fluorinated binaphthyl enantiomer, (R)-1, was applied as a chiral attempts to enlarge the temperature range of BPs have been
dopant to produce cholesteric blue phases (BPs). A fluorinated made, for example, dimeric LC molecules,3 hydrogen-bonded
nematic liquid crystal doped with (R)-1 showed lower critical chirality self-assembled LC complexes,4 a binaphthyl type LC molecule,5
to induce a BP and a larger temperature range of BPs than that doped gold nanorod dispersed BPs,6 an additive bend-core molecule,7
with a conventional chiral dopant.
dendron-stabilized BPs,8 and so on.9–12 However, an additional
dopant other than the basic components of the precursor or a
large change in chemical structure of the components oen
depresses the display performance. It is required to combine a
large temperature range of BPs before polymer-stabilization
with high electro-optical performances of BPs. In this study, we
focus our attention on a chiral dopant, which is a key material,
because large chirality and short helical pitch are needed to
induce the BP. If a chiral dopant is able to enlarge the
temperature range of BPs, both requirements of the BP
temperature range and electro-optic performance could be
satised simultaneously, without depressing the display
performance. Furthermore, a practical chiral dopant should be
compatible with uorinated host LCs because uorinated LCs
are indispensable for thin lm transistor (TFT) driving to retain
the high voltage holding ratio in display devices.
In our previous study, we synthesized a novel uorinated
chiral dopant, 6,60-uorinated binaphthyl, denoted as (R)-1
(Fig. 1), and found that it demonstrated a high helical twisting
power (HTP) and high miscibility with a uorinated nematic
LC.13 Here, we present the phase behavior and electro-optic
properties of the uorinated BPs induced by the chiral dopant
(R)-1, and compare them with BPs induced by a conventional
chiral dopant.
Blue phases (BPs),1–12,14–18,20,21 one of the chiral liquid crystal (LC)
phases, appear usually in a small temperature range between
chiral nematic (N*) and isotropic (Iso) phases, when the N*
phase has a relatively short helical pitch. There are three
different types of BPs: BP I, BP II, and BP III, in the order of
increasing temperature. The temperature range of a BP has
been reported to be enlarged remarkably in the polymer-stabi-
lized blue phase (PSBP),2 by which the BP state is retained at a
low temperature where the N* phase should inherently be the
stable phase in the absence of polymer-stabilization. The PSBP
is a promising candidate as a next-generation LC display (LCD)
material because it exhibits a fast electro-optic effect repre-
sented by an optical switching between isotropic and aniso-
tropic states, without surface alignment treatment. The PSBP
can be prepared by in situ photo-polymerization of an appro-
priate amount of monomers in a BP state. In the photo-poly-
merization process, a precursor solution basically consisting of
a host LC, chiral dopant, monomer, and photoinitiator is irra-
diated with UV light. The temperature of the precursor must be
adjusted very precisely to hold the BP state during the photo-
polymerization process. From the viewpoint of a “process
window” in the manufacturing of BP-LCDs, the temperature
range of the BP before photo-polymerization should be as large
as possible, even if the enlarging range is only 1 ꢀC. Several
aInterdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1
Kasuga-Koen, Kasuga, Fukuoka, 816-8580, Japan
bInstitute for Materials Chemistry and Engineering, Kyushu University, 6-1 Kasuga-
Koen, Kasuga, Fukuoka, 816-8580, Japan. E-mail: kikuchi@cm.kyushu-u.ac.jp
† Electronic supplementary information (ESI) available: The syntheses,
measurements, polarizing optical microscopy images, elastic constants data,
and electro optical properties. See DOI: 10.1039/c4tc01049c
Fig. 1 Chemical structures of 6,60-difluorinated binaphthyl derivatives.
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A
chiral dopant, 2,5-bis[40-(hexyloxy)-phenyl-4-carbonyl]- respectively, and the corresponding critical pitches were 428
1,4;3,6-dianhydride-D-sorbitol (ISO-(6OBA)2, the chemical and 491 nm, respectively. The critical chirality of BPs is gov-
structure of which is shown in Fig. S1 in the ESI†), was used as a erned by a thermodynamic energy balance between the N*
conventional dopant for comparison with (R)-1. A nematic phase and a BP. Low critical chirality corresponds to a readily
mixture of uorinated biphenyl cyclohexyl systems, JC-1041XX obtained BP. Furthermore, the temperature range of BPs in the
(Fig. S1 in ESI†), was used as the host uorinated LC. The JC-1041XX/(R)-1 mixture was larger than that in the JC-1041XX/
temperature range of BPs depends on the chirality, i.e., the ISO-(6OBA)2 mixture.
reciprocal of the helical pitch of the chiral LC phase.14–18 The
Due to Bragg diffraction from the cubic lattice, BPs exhibit
helical pitches for each sample were evaluated based on HTP, characteristic selective light reections in the visible wavelength
which was measured by the Grandjean-Cano wedge cell method range. The temperature dependences of the Bragg wavelengths
in the N* phase at 5 ꢀC below the clearing point, i.e. the LC for the mixtures of (a) JC-1041XX/ISO-(6OBA)2 ¼ 93.1/6.9 wt%
phase-isotropic liquid transition temperature Tc. The phase (sample 1) and (b) JC-1041XX/(R)-1 ¼ 91.5/8.5 wt% (sample 2)
transition behavior of BP samples was observed by polarizing were measured with a UV-Vis micro-spectrophotometer. The
optical microscopy (POM) and UV-Vis micro-spectrophotom- temperature range of BPs is known to depend on the chirality,
etry. The melting points of chiral dopants were measured by the helical pitch, and/or the lattice constant of the BP. To
differential scanning calorimetry. The physical properties of (R)- eliminate the effect owing to the differences in chirality, the
1 and ISO-(6OBA)2 are as follows: the HTPs (in the N* phase at helical pitches of the two BPs induced by (R)-1 and ISO-(6OBA)2
Tc – 5 ꢀC) are ꢁ51.5 and 58.4 mmꢁ1, the melting points are 185.1 were adjusted to be approximately identical by controlling the
ꢀ
and 88.3 C, respectively.
doping amounts of the chiral dopants in JC-1041XX. The
Fig. 2 shows the phase diagrams and the reduced phase temperature dependences of the Bragg wavelengths for BP I
transition temperatures as functions of the reciprocal of the (110) and for BP II (100) are shown in Fig. 3 for the JC-1041XX/
helical pitch in the N* phase of (a) JC-1041XX/ISO-(6OBA)2 and ISO-(6OBA)2 and JC-1041XX/(R)-1 mixtures. For the two samples
(b) JC-1041XX/(R)-1 mixtures. The reduced temperature T ꢁ Tc shown in Fig. 3, the Bragg wavelengths are close to each other,
represents the temperature difference from the clearing point. i.e. the lattice constants of both BPs are comparable. Never-
The transition temperatures were measured during the heating theless, the temperature ranges of the BPs of JC-1041XX/(R)-1
process to exclude the effect of supercooling, which is oen are larger than those of JC-1041XX/ISO-(6OBA)2. The results
observed for a phase transition from a BP to the N* phase. The shown in Fig. 2 and 3 clearly indicate that the chiral dopant (R)-
reciprocal of the helical pitch is indicative of the strength of 1 stabilizes a BP more effectively than ISO-(6OBA)2.
chirality. In general, BPs appear between N* and isotropic
liquids when the chirality is higher than a critical point. The the chiral dopant and the reciprocal of the Bragg wavelength in
critical reciprocals of the helical pitch for JC-1041XX/ISO- BP for the JC-1041XX/ISO-(6OBA)2 and JC-1041XX/(R)-1
(6OBA)2 and JC-1041XX/(R)-1 mixtures were 2.33 and 2.03 mmꢁ1
Fig. 4 shows the relationships between the concentration of
I
,
mixtures. A linear relation should be satised when a chiral
dopant is dissolved completely on a molecular level in a host
nematic LC. Although the JC-1041XX/ISO-(6OBA)2 mixture
showed good linearity over a ISO-(6OBA)2 concentration range
of 5 to 10 wt%, a slight downward deviation was observed at 12
wt%, as shown in plot 1 of Fig. 4. On the other hand, the JC-
1041XX/(R)-1 mixture obtained good linearity over the entire
concentration range of 5 to 12 wt%, as shown in plot 2 of Fig. 4.
These results indicate that the miscibility of (R)-1 is higher than
Fig. 2 Phase diagrams and the reduced phase transition temperatures Fig. 3 Temperature dependences of the Bragg wavelengths for (a)
as a function of the reciprocal of the helical pitch in the N* phase of (a) JC-1041XX/ISO-(6OBA)2 ¼ 93.1/6.9 wt% (sample 1) and (b) JC-
JC-1041XX/ISO-(6OBA)2 and (b) JC-1041XX/(R)-1 mixtures.
1041XX/(R)-1 ¼ 91.5/8.5 wt% (sample 2) mixtures.
6468 | J. Mater. Chem. C, 2014, 2, 6467–6470
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Fig. 5 Temperature dependences of K11 and K33 as well as the ratio
K
33/K11 for samples 3 and 4. The temperature Tc indicates the clearing
Fig. 4 Relationships between the concentration of the chiral dopant
and the reciprocal of the Bragg wavelength of BP I for JC-1041XX/ISO-
(6OBA)2 and JC-1041XX/(R)-1 mixtures.
point of sample or the nematic-coexisting phase transition
temperature of sample 4.
3
This tendency is in agreement with our experimental results.
that of ISO-(6OBA)2 in BP I. The temperature range of the BPs of Therefore, considering that the temperature range of BPs in JC-
JC-1041XX/ISO-(6OBA)2 is reduced in the high chiral region, as 1041XX/(R)-1 was enlarged by a decrease of K33 by doping with
shown in Fig. 2a. Generally, the temperature range of BPs (R)-1 is reasonable.
increases with increasing chirality. The low miscibility of ISO-
Sample 4 showed a relatively large temperature range, over
(6OBA)2 to JC-1041XX could be responsible for this unusual which the nematic and isotropic phases coexisted as a result of
reduction of the BP temperature range at high chirality. On the the addition of rac-1 (Fig. S5 in ESI†). The coexisting phase
ꢀ
other hand, such a phenomenon was not observed for the JC- ranged over a span of 5.8 C, which is much wider in compar-
1041XX/(R)-1 mixture because of the sufficiently high miscibility ison with the host LC (sample 3). Producing a disclination line
of (R)-1 to JC-1041XX.
requires excess thermal energy below the clearing point because
The thermodynamic stability of BPs is theoretically shown to the orientational order within the disclination cores is low like
depend signicantly on the value of the Frank elastic constants, an isotropic phase.18,20 It has been reported that the orienta-
splay (K11), twist (K22), and bend (K33), of the host LC materials. tional order in a LC is associated with the ratio of the length L to
Therefore, evaluating the inuence of doping with (R)-1 on the the width D (L/D) of the constituent molecules, and the orien-
value of the Frank elastic constants of the host LC is mean- tational order decreases as the L/D of a LC molecule decreases.22
ingful. However, the elastic constants are typically measured The L/D of the compound rac-1, owing to its wide shaped
with uniformly aligned LCs, and therefore these constants can binaphthyl skeleton, is smaller than both that of the host LC
be difficult to determine for a highly twisted LC. To overcome molecule and ISO-(6OBA)2. Hence, the addition of the chiral
this difficulty, we synthesized the racemic binaphthyl dopant (R)-1 having a small L/D might be able to facilitate the
compound, rac-1 (for details, see ESI†), that has an identical formation of disclination lines below the clearing point and to
chemical structure to (R)-1 (Fig. 1), but which does not induce lower the free energy of the BPs. This could be a possible
twist alignment in the LC. We measured the elastic constants, mechanism of the stabilization effect by addition of (R)-1. When
K11 and K33, of the pure JC-1041XX without dopant (sample 3, a dendron molecule with a small L/D is added to a BP sample,
TNI ¼ 94.3 ꢀC) and the JC-1041XX/rac-1 ¼ 91.5/8.5 wt% mixture the temperature ranges of the BPs and nematic-isotropic coex-
(sample 4) by means of the capacitance method.19 The phase isting phases is enlarged with an increasing amount of the
transition temperatures of sample 3 were 59.7 ꢀC (N to N + Iso.) added dendron molecule.8
ꢀ
and 65.5 C (N + Iso. to Iso), which were observed by POM as
As shown in Fig. 2, the clearing point Tc of JC-1041XX/(R)-1 is
discussed later. Fig. 5 shows the temperature dependences of lower than that of JC-1041XX/ISO-(6OBA)2, suggesting that (R)-1
K11 and K33 as well as their ratio K33/K11 for samples 3 and 4. The has the larger effect of lowering the order parameter of the host
differences in the values of K11 between samples 3 and 4 were LC than ISO-(6OBA)2. The blue phase must coexist with dis-
relatively small. In contrast, the respective values of K33 were clinations to form the double twist alignment. The order
very different, wherein the K33 values of sample 4 were lower parameter in the core of disclination should be lowered as
than those of sample 3. The ratio K33/K11 of sample 4 was also mentioned above. Therefore, the effect of lowering the order
smaller than that of sample 3 over the entire measuring parameter by (R)-1 might be also responsible for the observed
temperature range. Thus, the chiral dopant (R)-1 is demon- larger temperature range of blue phases.
strated to more readily cause bend deformation of LC directors.
The electro-optical properties of the BPs of samples 1 and 2
Assuming that a similar situation could occur for local directors were investigated. The transmittances of the BPs were measured
in the BPs of JC-1041XX/(R)-1 is reasonable. The thermody- upon an applied electric eld driven by a 1 kHz square wave
namic stability of BPs has been shown to theoretically and with comb type electrodes under crossed Nicols. Response
experimentally be greatly enhanced when K33 is smaller.7,8,20,21 times at the voltage at which the BP I exhibited maximum
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Table 1 Electro-optical properties of samples 1 and 2
Notes and references
Kerr coefficient/10ꢁ11 m Vꢁ2
srise/ms
sdecay/ms
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Sample 1
Sample 2
9.5
10.5
966
442
455
406
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transmittance (Vmax) and Kerr coefficients were measured at the
intermediate temperature between TN*–BP I and TBP I–BP II for
samples 1 and 2. The results are summarized in Table 1. The
Kerr coefficients and the response times depend strongly on the
helical pitch of BPs. Therefore, by controlling the doping
concentration, the helical pitches of samples 1 and 2 were
adjusted to be nearly identical. The maximum transmittance,
Kerr constants, and response time for the voltage-off state
(sdecay) were comparable between the two samples, whereas the
response time of the voltage on state (srise) of the BP I of sample
2 was less than half that of sample 1, as shown in Table 1. Thus,
the electro-optical performance of JC-1041XX/(R)-1 is concluded
to be equal to or greater than that of the conventional system,
JC-1041XX/ISO-(6OBA)2.
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Conclusions
A novel chiral uorinated binaphthyl derivative, (R)-1, which
was previously synthesized by us, was applied as a chiral dopant
to produce uorinated cholesteric blue phases. A lower critical
chirality to induce a blue phase and a larger temperature range
of blue phases were produced in a chiral uorinated nematic
liquid crystal doped with (R)-1. Compared with a conventional
sorbitol type chiral dopant, (R)-1 showed a high ability to
stabilize blue phases. To understand the stabilizing effect of (R)-
1, Frank elastic constants were measured with a mixture of the
uorinated nematic liquid crystal and rac-1. The racemic
binaphthyl compound rac-1 induced a low elastic constant of
bend deformation and an extended temperature range over
which the nematic phase coexisted with the isotropic phase,
and these could be possible mechanisms of the high stabilizing
effect of (R)-1 on blue phases. Most electro-optical properties of
JC-1041XX/(R)-1 were comparable with those of the conven-
tional materials, but the rise response was faster.
ˆ
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This work was partially supported by JSPS KAKENHI Grant
Numbers 21245037, 25248021, and 24750130, Grant-in-Aid for
Scientic Research (no. 22107008) on the Innovative Areas:
“Fusion Materials” (Area no. 2206) and The Global COE
program, Novel Carbon Resource Sciences, Kyushu University.
22 H. Kimura, J. Phys. Soc. Jpn., 1974, 36, 1280–1287.
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