Biphenyl derivative stabilizing blue phases

Article abstract of DOI:10.1039/c1jm13299g

After preparing a biphenyl derivative possessing two 2,3-difluoro-1,4- diphenylbenzene units, we investigated its phase transition behaviour. It showed a phase sequence of isotropic liquid-nematic phase-anticlinic smectic C phase. A mixture with a chiral

Full text of DOI:10.1039/c1jm13299g

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PAPER
Biphenyl derivative stabilizing blue phases
a
Yutaro Kogawa, Tetsu Hirose and Atsushi Yoshizawa
b
a
*
Received 14th July 2011, Accepted 28th September 2011
DOI: 10.1039/c1jm13299g
After preparing a biphenyl derivative possessing two 2,3-difluoro-1,4-diphenylbenzene units, we
investigated its phase transition behaviour. It showed a phase sequence of isotropic liquid–nematic
phase–anticlinic smectic C phase. A mixture with a chiral additive with a high twisting power exhibited
a cubic blue phase and/or an amorphous blue phase III (BPIII). The compound doped with 10 wt% of
ꢀ
the chiral additive showed a phase sequence of Iso 85.4 C BPIII 70.6 ꢀC cubic BP 47.3 ꢀC chiral
nematic 37.3 ꢀC unidentified smectic phase. We investigated electro-optical switching in BPIII and in
the cubic BP of the mixture. Transmittance without an applied electric field in the BPIII was 0%,
whereas that in the cubic BP was 4.6%. Threshold and driving voltages in the BPIII are lower than in
the cubic BP. Furthermore, the BPIII exhibits no voltage-induced hysteresis effects on transmittance.
Amorphous BPIII has superior potential for use in next-generation displays.
Blue phases are of particular interest because they have a fluid
lattice whose structure is stabilized by lattice defects. Believed to
consist of double twist cylinders, they are classified into three
categories depending on the cylinders’ packing structure: blue
phase I (BPI), blue phase II (BPII), and blue phase III (BPIII).1
The packing structure of BPI is a body-centred cubic structure,
BPII is a simple cubic structure.2,3 In contrast, BPIII is expected
to consist of double-twisted cylinders with arbitrary orienta-
tion.2,4,5 Usually, blue phases are found in an extremely narrow
temperature range (ca. 1 K) between the isotropic liquid and the
chiral nematic (N*) phase of sufficiently short pitch. Blue phases
are potentially useful for application as fast light modulators or
tunable photonic crystals, but their narrow temperature range is
a critical problem. Therefore stabilizing the blue phases has
attracted much attention.6–18 The most successful approach is
polymer stabilization, as reported by Kikuchi et al.6 Specific
polymer networks can stabilize the lattice defects of a BPI. The
temperature range of the polymer-stabilized BPI is extended to
more than 60 K. Fast electro-optical switching with a response
time of 10ꢁ4 s was demonstrated in the polymer-stabilized BPI.
Blue phases have several advantages: fast response speed and
optical isotropy. Therefore a blue-phase LCD mode using
polymer-stabilized BPI was developed in earlier studies.19–22
However, critical problems such as high operating voltage,
hysteresis, residual transmittance, and stability remain as
necessary improvements for application to display devices.
Particularly, voltage-induced hysteresis affects the accuracy of
gray scale control and should therefore be eliminated. This
report describes electro-optical switching in an amorphous
BPIII.23 Actually, several groups have investigated BPIII
switching,15,18,24 but no report describes comparison of electro-
optical properties between a BP with a cubic structure (cubic BP:
BPI or BPII) and amorphous BPIII using a single compound or
mixture because no single system exhibits both those blue phases
with wide temperature ranges. Elucidation of the influence of the
molecular geometry on the stability of blue phases remains in its
infancy. A recent report describes that a binaphthyl derivative
possessing two 2,3-difluoro-1,4-diphenylbenzene units exhibits
a cubic blue phase with a temperature range of about 30 K.25
Herein a newly designed biphenyl derivative possessing two 2,3-
difluoro-1,4-diphenylbenzene units is proposed. It exhibits cubic
BP and/or amorphous BPIII in mixtures with a chiral compound.
We discuss their mutual differences in electro-optical properties.
Experimental
Materials
Spectroscopic analysis. Purity of the final compound was also
checked using elemental analysis (EA 1110; CE Instruments
Ltd.). Infrared (IR) spectroscopy (FTS-30; Bio-Rad Laborato-
ries Inc.) and proton nuclear magnetic resonance (1H NMR)
spectroscopy (JNM-ECA500; JEOL) elucidated the structure of
the final product.
Preparation of materials. For use in this study, 2,3-difluoro-4-
(4-hexylphenyl)-1-(4-hydroxyphenyl)benzene was purchased
from Midori Kagaku Co. Ltd.
aDepartment of Frontier Materials Chemistry, Graduate School of Science
and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki, Aomori,
036-8561, Japan. E-mail: ayoshiza@cc.hirosaki-u.ac.jp
bTohoku Chemical Corporation, 1-3-1, Kanda, Hirosaki, Aomori, 036-
8655, Japan
2,20-Bis{6-[4-(4-(4-hexylphenyl)-2,3-difluorophenyl)phenyl-
oxycarbonyl]hexyloxy}-1,1-biphenyl (1). To a solution of 2,20-
biphenol (0.37 g, 2.0 mmol) and ethyl 7-bromoheptanoate (1.0 g,
19132 | J. Mater. Chem., 2011, 21, 19132–19137
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4.0 mmol) in cyclohexanone (20 ml) was added potassium
carbonate (0.58 g, 3.0 mmol). The reaction mixture was stirred
the cell under parallel polarizers. Transmittance with 0% was
calibrated by that of the cell under crossed polarizers. The
distance between the electrodes was 10 mm and the cell gap was
maintained at 12 mm by spacers. The cells were kindly supplied
by Chisso Petrochemical Corp.
ꢀ
at 140 C for 8 h. After filtration of the precipitate, the solvent
was removed by evaporation. The residue was purified using
column chromatography on silica gel with a toluene and ethyl
acetate (4 : 1) mixture as the eluent, giving 2,20-bis[6-(ethyl-
oxycarbonyl)hexyloxy]biphenyl as a colourless liquid; yield 0.87
g (87%).
Results and discussion
First, 2,20-bis[6-(ethyloxycarbonyl)hexyloxy]biphenyl (0.60 g,
1.2 mmol) was added to a solution of NaOH (0.12 g, 3.0 mmol) in
an ethanol (30 ml) and water (10 ml) mixture. The resulting
solution was refluxed with stirring for 4 h. After removal of the
ethanol by evaporation, the residue was acidified using aq. HCl.
The solution was extracted with dichloromethane. The organic
layers were combined, dried over magnesium sulfate, filtered,
and evaporated. The residue was washed with hexane to give
7,70-(biphenyl-2,20-diylbisoxy)bisheptanoic acid as a white solid;
Liquid crystalline properties
The biphenyl derivative, 2,20-bis{6-[4-(4-(4-hexylphenyl)-2,3-
difluorophenyl)phenyloxycarbonyl]hexyloxy}-1,1-biphenyl (1),
was prepared as described below through synthesis. 2,20-Biphe-
nol was treated with ethyl 7-bromoheptanoate in the presence of
potassium carbonate. The obtained ester was hydrolyzed to give
the corresponding carboxylic acid. The intermediate was treated
with 4-(4-(4-hexylphenyl)-2,3-difluorophenyl)phenol in the
presence of DCC to give the target compound.
ꢀ
yield 0.53 g (92%); mp 70.4–71.6 C.
To a solution of 7,70-(biphenyl-2,20-diylbisoxy)bisheptanoic
acid (0.18 g, 0.40 mmol) in dichloromethane (30 ml), 2,3-
difluoro-4-(4-hexylphenyl)-1-(4-hydroxyphenyl)benzene (0.29 g,
0.80 mmol), N,N0-dicyclohexylcarbodiimide (0.17 g, 0.80
mmol), and 4-(N,N-dimethylamino)pyridine (0.009 g, 0.08
mmol) were added. The resulting solution was stirred at room
temperature for 20 h. Precipitated materials were removed by
filtration. After removal of the solvent by evaporation, the
residue was purified using column chromatography on silica gel
with dichloromethane as the eluent and then recrystallized with
an ethanol and ethyl acetate (1 : 1) mixture, giving the desired
product as a white solid; yield 0.30 g (64%); mp 115.0–116.1 ꢀC.
1H NMR (500 MHz, solvent CDCl3, standard TMS) d/ppm:
7.59 (dd, 4H, Ar-H, J ¼ 8.6 Hz, 1.2 Hz), 7.50 (dd, 4H, Ar-H, J
¼ 8.0 Hz, 1.2 Hz), 7.29 (d, 4H, Ar-H, J ¼ 8.6 Hz), 7.32–7.21 (m,
8H, Ar-H), 7.18 (d, 4H, Ar-H, J ¼ 8.6 Hz), 6.998 (t, 1H, Ar-H,
J ¼ 7.5 Hz), 6.996 (t, 1H, Ar-H, J ¼ 7.5 Hz), 6.96 (d, 2H, Ar-H,
J ¼ 8.6 Hz), 3.94 (t, 4H, -OCH2-, J ¼ 6.3 Hz), 2.67 (t, 4H, Ar-
CH2-, J ¼ 7.7 Hz), 2.52 (t, 4H, -OCOCH2-, J ¼ 7.7 Hz), 1.72–
1.33 (m, 32H, aliphatic-H), 0.90 (t, 6H, aliphatic-H, J ¼ 6.9 Hz).
IR (KBr) nmax/cmꢁ1: 2929, 2855, 1752, 1460, 1209. Elemental
anal. calcd for C74H78O6F4: C 78.01, H 6.90. Found: C 77.94, H
6.87%.
Phase transition behaviour was investigated using polarized
optical microscopy and DSC. On cooling, compound 1 showed
ꢀ
the following phase sequence: isotropic liquid 106.1 C (1.2 kJ
molꢁ1) nematic (N) phase 69.0 ꢀC (0.8 kJ molꢁ1) anticlinic
ꢀ
smectic C (SmCanti) phase ꢁ1.4 C glass. The melting temper-
ꢀ
ature was 113 C. The SmCanti phase was identified by optical
microscopy, i.e., the texture of the homeotropically aligned
sample was a schlieren texture that possessed singularities with
two (s ¼ 1/2) and four (s ¼ 1) brushes (Fig. 1). The two-brush
singularities were not able to be formed in the synclinic phase,
but they can be generated with anticlinic ordering.26 Dielectric
anisotropy of compound 1 in the N phase was obtained as ꢁ1.1
ꢀ
at 80 C.
We investigated the transition behaviour of binary mixtures of
compound 1 and a chiral compound ISO-(6OBA)2. The chiral
compound possesses high twisting power27 and is well known to
induce blue phases in the mixture with a nematic liquid
crystal.6,10,15,18,28
Liquid-crystalline and physical properties. The initial phase
assignments and corresponding transition temperatures for the
final compound were determined using thermal optical micros-
copy with a polarizing microscope (BX-51; Olympus Optical
Co. Ltd.) equipped with a temperature control unit (LK-
600PM; Japan High Tech). The temperatures and enthalpies of
transition were investigated using differential scanning calo-
rimetry (DSC, DSC 6200; Seiko Instruments Inc.). Dielectric
measurements were performed using an impedance analyzer
(Hewlett Packard HP 4284A) with a temperature control unit at
a frequency of 1 kHz. Homogeneous and homeotropic config-
uration cells (5.0 mm) purchased from E.H.C. were used for the
measurements.
Optical transmittance as a function of applied electric AC field
at 10 Hz was observed for a sample contained in the region
between the comb-type interdigitated electrodes under crossed
polarizers. Transmittance with 100% was calibrated by that of
Fig. 1 Photomicrograph of the SmCanti phase of a homeotropically
aligned sample of compound 1 at 60 ꢀC.
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Fig. 3 Cooling DSC thermogram of a mixture of ISO-(6OBA)2 (13 wt%)
and compound 1 (87 wt%) at a scanning rate of 1 ꢀC minꢁ1
.
Fig. 2 shows the binary phase diagram on cooling between
ISO-(6OBA)2 and compound 1 obtained using optical micros-
copy. All mesophases were monotropic. The SmX phase is an
unidentified smectic phase. Neither ferroelectric nor antiferro-
electric switching was observed in the SmX phase. The cooling
rate was 0.1 ꢀC minꢁ1 from isotropic liquid to BP; then the
cooling rate was changed to 2 ꢀC minꢁ1. For mixtures containing
7–13 wt% of ISO-(6OBA)2, blue phases were found to be
induced. In a mixture of ISO-(6OBA)2 (7 wt%) and compound 1
(93 wt%), a platelet texture with various colours which is usually
observed in blue phases with a cubic structure appeared at 89.5
ꢀC; then it changed to a typical N* texture at 53.6 ꢀC. In
a mixture of ISO-(6OBA)2 (13 wt%) and compound 1 (87 wt%),
a blue colour of low birefringence appeared at 69.2 ꢀC. The blue
colour phase was clearly observed in uncovered regions of the
mixture. The blue phase showed fluidity and did not appear as
platelets. These observations indicate that the phase is an
amorphous BPIII.ꢀWhen cooled further, the BPIII changed to an
N* phase at 43.4 C. Fig. 3 depicts a DSC thermogram of the
mixture on cooling from isotropic liquid. It is particularly
interesting that a mixture of ISO-(6OBA)2 (10 wt%) and
compound 1 (90 wt%) exhibited both amorphous BPIII and
cubic BP. Fig. 4(a) and (b) respectively show textures of BPIII
and cubic BP of the mixture. Fig. 5 depicts a DSC thermogram of
the mixture on cooling from isotropic liquid. The phase transi-
tion temperatures were theꢀfollowing: Iso 85.4 ꢀC BPIII 70.6 ꢀC
Fig. 4 Photomicrographs of (a) the BPIII at 73.6 ꢀC and (b) the cubic BP
at 65.0 ꢀC of a mixture of ISO-(6OBA)2 (10 wt%) and compound 1 (90 wt
%).
ꢀ
cubic BP 47.3 C N* 37.3 C SmX. No transition enthalpy was
Fig. 5 Cooling DSC thermogram of a mixture of ISO-(6OBA)2 (10 wt%)
and compound 1 (90 wt%) at a scanning rate of 1 ꢀC minꢁ1
detected at the BPIII to cubic BP transition. The temperature
ranges of the BPIII and the cubic BP were, respectively, 14.8 K
and 23.3 K. Therefore we can investigate electro-optical prop-
erties in amorphous BPIII and cubic BP of the single mixture.
Compound 1 can exist in either of two conformations: cis or
trans with respect to the C1–C10 axis of the biphenyl moiety.
Molecular-mechanical calculations using MOPAC-6/PM3 of
.
Fig.
2
Binary phase diagram of mixtures of ISO-(6OBA)2 and
Fig. 6 MOPAC models for compound 1 with (a) cis conformation and
(b) trans conformation.
compound 1.
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compound 1 suggest that the cis conformation (Fig. 6(a)) is more
stable than the trans one (Fig. 6(b)). The energy difference
between the cis and trans conformers of compound 1 was esti-
mated to be 43 kJ molꢁ1. If compound 1 forms the trans
conformation, then the I–N transition temperature should be
much higher than the observed value. For these reasons, we
assumed the cis conformer for compound 1. With respect to
chiral effects of the binaphthyl derivatives on blue phase stabi-
lization, we made the following interpretation.25,29 The
binaphthyl derivative exists as a cisoid form, which has two
helical origins, the asymmetric axis and the twisted conformation
of the mesogenic units. Their coexistence is able to stabilize the
double-twist structure in the blue phase. In the present system,
chiral transfer from ISO-(6OBA)2 to compound 1 via core–core
interaction can induce twisting of the biphenyl axis, which
produces a twist conformation of the two terphenyl mesogenic
units, as shown in Fig. 7.30,31 Therefore compound 1 has two
origins for the twisting power: a twist conformation of the
biphenyl moiety and that of the mesogenic units, which can
stabilize the blue phases.
Fig. 8 Optical transmittances of the mixture ISO-(6OBA)2 (10 wt%) and
compound 1 (90 wt%) as a function of AC field at a frequency of 10 Hz in
(a) BPIII at 80 ꢀC and (b) cubic BP at 60 ꢀC. The ascending and
descending cycle is depicted in Fig. 8(a). The only ascending process is
shown in Fig. 8(b). Open circles and crosses respectively represent
ascending and descending processes.
Electro-optical studies
We investigated electro-optical properties in the BPIII and the
cubic BP of the mixture of ISO-(6OBA)2 (10 wt%) and
compound 1 (90 wt%). Optical transmittance, as a function of
applied electric AC field at 10 Hz, was observed for a sample
contained in the region between comb-type interdigitated elec-
trodes under crossed polarizers. Fig. 8(a) and (b) respectively
portray the voltage-dependent transmittance curve in the BPIII
at 80 ꢀC and that in the cubic BP at 60 ꢀC. In BPIII (Fig. 8(a)),
the transmittance without an electric field was 0%. The trans-
mittance increased concomitantly with the increase of the electric
field. With an applied field of 9 V mmꢁ1, the texture showed
a homogeneous bright state with a transmittance of 80%. The
transmittance decreased with the decrease of the electric field.
The backward curve was the same as the forward curve. It is free
from both hysteresis and residual transmittance. According to
Wu et al.,32 the compact packing in a BP with a cubic lattice
structure hinders the formation of the BP structure during the
voltage descent process. The absence of a lattice structure in the
BPIII produces the hysteresis-free switching. The extinction
direction was obtained when the sample with the saturated
transmittance was rotated 45ꢀ. Transmittance of the dark state
was 1.5%. These observations indicate that the electric-field-
induced bright state is the N phase. We estimated the response
times in rise and decay processes. The rise and decay times were
10 s and 8 s, respectively. Therefore the transmittance change for
BPIII shown in Fig. 8(a) is attributed to electric-field-induced
phase transition between BPIII and N. There is another possi-
bility that the present electro-optical response is due to reor-
ientation of the local director. In that case, the decay time is
thought to be much faster than 1 s. In the cubic BP (Fig. 8(b)),
transmittance without an electric field was 4.6%. Although the
transmittance increased concomitantly with the increase of the
electric field, it did not reach a saturated value with an applied
field of 12 V mmꢁ1. Threshold and saturated voltages in the cubic
BP were higher than those in the BPIII. We were not able to
observe the backward process because the transmittance under
an AC field showed time-dependent decay. Such decay was not
observed in BPIII. When the sample with the transmittance of
70% was rotated, the texture did not change, indicating that the
electric-field-induced bright state is the N* phase. The increase in
the transmittance for cubic BP shown in Fig. 8(b) is thought to
result from cubic BP to N* phase transition.
Time-dependent transmittance under the AC field of 12 V
mmꢁ1 was observed. The results are portrayed in Fig. 9. In the
ꢀ
BPIII at 80 C (Fig. 9(a)), the transmittance increased with the
increase of time, reaching the saturated value. The saturated
transmittance did not change with further increase of time.
However, in the cubic BP at 60 ꢀC (Fig. 9(b)), the transmittance
reached the saturated value of 70%; then it decreased with the
increase of time. The transmittance of 1.5% at 30 s is lower than
Fig. 7 Schematic model for chirality transfer from a core part of ISO-
(6OBA)2 to a biphenyl unit of compound 1.
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dimensional order is more stable against an electric field than
cubic BP. The instability against the electric field for the cubic BP
is one of the reasons to generate the hysteresis of the present
cubic BP. The rise time in the BPIII is almost same to that in the
cubic BP. In the present system, the response speed is very slow
because of the negative dielectric anisotropy of compound 1. The
response time in BPIII can be readily shortened to less than 1 ms
at room temperature by using materials with positive dielectric
anisotropy.33
Conclusions
We designed a new biphenyl derivative possessing 2,3-difluoro-
1,4-diphenylbenzene units and found that it exhibits the N and
SmCanti phases. A mixture with a chiral additive exhibited
a cubic blue phase and/or an amorphous BPIII with a wide
temperature range. Comparing the voltage-dependent trans-
mittance in the BPIII and the cubic BP of a single mixture, the
BPIII was found to have the following advantages: (1) free from
hysteresis, (2) lower transmittance without an electric field, (3)
lower threshold and saturated voltages, and (4) higher stability
against an electric field. BPIII with the same symmetry as
isotropic liquid has favourable characteristics for application to
display devices: it is optically isotropic, hysteresis free, and stable
in an electric field.
Fig. 9 Optical transmittances of the mixture ISO-(6OBA)2 (10 wt%) and
compound 1 (90 wt%) as a function of time under an AC field of 12 V
mmꢁ1 at a frequency of 10 Hz in (a) BPIII at 80 ꢀC and (b) cubic BP at
60 ꢀC.
Acknowledgements
We thank Chisso Petrochemical Corporation for providing the
cells used for this study. This work was partially supported by
a Grant-in-Aid for Scientific Research (B) from the Japan
Society for the Promotion of Science (no. 22350078) and a grant
for Hirosaki University Institutional Research.
that without an electric field. After eliminating the electric field
and then re-applying it to the sample, no electro-optical switch-
ing occurred. This electro-optical behaviour in the cubic BP
occurred under any electric field with a magnitude of 6–12 V
mmꢁ1. Phase transition temperatures of the sample after applying
electric field were identical to those of the virgin sample. One
possible explanation for the unusual decrease in the trans-
mittance is an electric-field-induced phase transition from N* to
isotropic liquid. The electro-optical responses in BPIII and cubic
BP are reproducible. Therefore the marked difference in the
electro-optical response between BPIII and cubic BP is attributed
to the difference in the blue phase structure between them.
According to earlier studies of electric field effects in cubic blue
phases,1 an increasing electric field initially lengthens the blue
phase lattice parameter and causes birefringence. With higher
fields, the blue phases may transform between themselves, to the
helical phase, and ultimately to the nematic phase. Because BPIII
has no lattice structure, we infer that the rise process in BPIII
includes two transformations: a double twisted structure to
a single twisted structure breaking their cylindrical structure
(BPIII to N*) and the single twisted structure to the nematic
structure (N* to N). With respect to the cubic BP, short-range
correlation due to the lattice structure can remain under the
electric field. A double twisted structure might transform to an
unstable single twisted structure with the short-range correlation.
The activation energy of transition from the electric-field-
induced N* state to the isotropic liquid for cubic BP is thought be
lower than that from the electric-field-induced N state to the
isotropic liquid for BPIII. Further investigation is necessary to
clear the mechanism. Amorphous BPIII without three-
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J. Mater. Chem., 2011, 21, 19132–19137 | 19137