Ultrashort helix pitch antiferroelectric liquid crystals based on chiral esters of terphenyldicarboxylic acid

Article abstract of DOI:10.1039/c6tc04087j

Antiferroelectric liquid crystals (AFLCs) with a nanoscale helix pitch (<100 nm) were revealed in a composition containing achiral smectic-C biphenylpyrimidines and two non-mesogenic chiral dopants: p-terphenyldicarboxylates of chiral (2S)-1,1,1-trifluorooctane-2-ol and (S,S)-1,1,1-trifluorooctan-2-yl 2-hydroxypropanoate. The prepared multicomponent AFLCs exhibit two electro-optical effects, which are interrelated with the chemical structures of the mixture's components. The first effect is hysteresis free under special voltage waveform U-shaped switching, which exhibits an electro-optical response similar to that of nematic liquid crystals (NLCs) but around 1-2 orders of magnitude faster. Secondly, the deformed helix antiferroelectric liquid crystal (DHAFLC) effect has been investigated. The observed temperature independence of the electro-optical parameters, in a certain temperature range, combined with a fast U-shape response or with DHAFLC, adds a great value for applications.

Full text of DOI:10.1039/c6tc04087j

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V. V. Vashchenko, V. V. Mikhailenko, A. I. Krivoshey, V. A. Barbashov, L. Shi, A. K. Srivastava, V. Chigrinov
and H. S. Kwok, J. Mater. Chem. C, 2016, DOI: 10.1039/C6TC04087J.
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Ultrashort helix pitch antiferroelectric liquid crystals based on chiral esters of
terphenyldicarboxylic acid
a,b
a,c
c
c
Received 00th January 20xx,
Accepted 00th January 20xx
E.P. Pozhidaev, V.V. Vashchenko, V. V. Mikhailenko, A.I. Krivoshey,
b
a
a
a
a
V.A. Barbashov, L. Shi, A.K. Srivastava,* V.G. Chigrinov, and H.S. Kwok
DOI: 10.1039/x0xx00000x
Antiferroelectric liquid crystals (AFLCs) with nanoscale helix pitch (<100 nm) were revealed in a composition containing
www.rsc.org/
achiral smectic-C biphenylpyrimidines and two non-mesogenic chiral dopants: p-terphenyldicarboxylates of chiral (2S)-
1
,1,1-trifluooctane-2-ol and (S,S)-1,1,1-trifluorooctan-2-yl 2-hydroxypropanoate. The prepared multicomponent AFLCs
exhibit two electro-optical effects, which are interrelated with the chemical structures of the mixtures components. First
effect is hysteresis free under a special voltage waveform U-shaped switching, which exhibits the electro-optical response
similar to nematic liquid crystals (NLCs) but around 1 - 2 orders of magnitude faster. Second, deformed helix
antiferroelectric liquid crystal (DHAFLC) effect has been investigated. The observed temperature independence of the
electro-optical parameters, in a certain temperature range, combined with fast U-shape response or with DHAFLC, adds a
great value for applications.
5
,12,13,19
contrary, the so-called orthoconic AFLCs,
Introduction
provide excellent optical quality of AFLCs display cells
due to 45° tilt angle, but their antiferroelectric CA*
phase at room temperature is rather slow in electro-
AFLCs are innovative electro-optical materials with
great potential for various display and photonic
4,8,20-22
1
-5
optical response
in electro-optical response can be achieved in this case
only by using very intricate driving voltage
waveform or asymmetric switching due to different
. Compensation of the hysteresis
applications.
In spite of AFLCs were a long ago
6
-8
employed to develop prototypes of displays, and
great success in AFLC material science for subsequent
a
1
4
1
,9-14
years,
displays based on such materials are still far
1
2
3
,14
anchoring surfaces,
which is very difficult to
from being commercial.
Main inherent drawbacks
implement in display technology.
hampering AFLCs commercial application are the
sensitivity of the uniform alignment to mechanical
shock, very pronounced double W-shaped hysteresis
loop, and the dark state problem (or simply insufficient
It is also worth to note, that most of known AFLC
devices have been targeted for using in SSFLC mode
where rather long helix pitch is preferable, typically
1
,2,4,5
1
,15
from few µm.
But typical helix pitch values of most
alignment quality of AFLC display cells).
4
of AFLC materials are around of 1 µm, and special
efforts were applied to increase the pitch in order to
harmonize it with LC cell thickness. On the contrary,
the mentioned pitch values are too large for effective
applications using short-pitch electrooptical modes,
e.g. as deformed helix ferroelectric (DHF) effect in
Unfortunately, even modern AFLC materials are not
free from all of these disadvantages. Thus, in the
3
,16,17
including those based on
are insensitive to mechanical
polymer-stabilized AFLC
1
bent-core molecules
1,18
1
1
shock and hysteresis free . However, the contrast
3
ratio of the electro-optical modulation is rather low. In
2
4
SmC* phase.
Hence, application potential of
ultrashort helix pitch AFLCs has not been considered so
far.
Here we report on design and synthesis of new
chiral trifluoromethylated esters of terphenyl-di-
carboxylic acid which are in mixtures with biphenyl-
pyrimidine host allowed to elaborate ultrashort AFLC
materials exhibiting DHF and hysteresis free U-shaped
electro-optical switching.
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we envisaged further increasing of the
s
P in LC
Materials and methods
mixtures. Because of the natural concentration limit for
the compatibility of an individual dopant with the SmC
host, the required CDs content can be raised in the
mixtures of several chiral compounds of the same sign
of spontaneous polarization as well as helical twisting.
Thus, it is reasonable to choose S-FOTDA as the first
chiral dopant. With the additional dopant, we have
attempted to increase P by combining the CF function
Structural design of chiral dopants
Structural requirements to chemical compounds
exhibiting AFLC ordering hitherto are formulated only
in very general terms and mainly with respect to
1
,2,4,25,26
individual mesogens (Fig. 1).
Thus, anticlinic
phases are often formed by bent-core or by so-called
1
1,18,27
bi-mesogens (Figs. 1a and 1b).
However, because
s
3
of their high viscosity these types of structures will not
be further considered.
with chiral S-lactate unit, which is also often deployed
in the structure of the FLC dopants particularly in the S-
lactate esters of terphenyl-dicarboxylic acid (LACT
3
)
7,39,40
.
The designed chemical formula of the new
dopant is given below (see Fig. 2). The proposed
structures of the new dopants, also follows the general
tendency of the anticlinic packing appearance (see
above), due to a shift of the fluorinated substituent
away from the rigid terphenyl core, thereby closing
Fig. 1 Schematic drawing of molecule types prone to formation of anticlinic
packing.
them to the type c (see Fig. 1).
One of the general structure requirements for
single-core LC substances (Fig. 1c) which are prone to
anticlinic packing is presence at least three rings in the
2
,26,28
central core.
Besides, the probability of formation
of anticlinic smectic arrangement typically increases
with enhancement of inter-layer interactions. It can be
achieved by means of modification of the terminal tails
with specifically interacting groups, often highly polar
group (SG), which is separated from the core with
several methylene units. Among these “specific
1
0,12,13,29
9,28,30
Fig. 2. The design of new chiral dopant of LACTAF-6 series.
groups”, ether or ester function,
1
siloxanes,
and trifluoromethyl
9,31,32
partially fluorinated alkyls
1
2,13, 24,33
unit often occurs.
In our structural design, the well-known chiral
It is worth to point that the presence of two chiral
centers at each wing of a LACTAF molecules results in
appearance as four possible diastereomers combined
in two mirror pairs (S,S and R,R) and (R,S and S,R).
Combining the natural S-lactate with synthetic S- or R-
3
4
diesters of p-terphenyl dicarboxylic acid have been
chosen as a basic motif. In particular, we exploited
recently described (S,S)-bis-(1,1,1-trifluorooct-2-yl)-
3
5
triflouromethylalkanols
diastereomers of each pair mentioned above have
been prepared and studied (S,R-LACTAF-6 S,S-LACTAF-
6).
the
representative
4,4''-terphenyl dicarboxylate (S-FOTDA-6):
O
O
O
O
,
F
3
C
CF
3
(S)
(
S)
C H
6 13
6 13
C H
As non-chiral LC host, the mixture (M1) of two
4
1
known biphenylpyrimidines has been chosen as a
wide-range SmC non-chiral host (see Fig. 3):
which is non-mesogenic itself but induces chiral
smectic C* phase in its mixtures with two or three-ring
achiral SmC hosts with the spontaneous polarization P
2
s
1
10–160 nC/cm and sub-wavelength helix pitch p
0
3
5-38
estimated ≈ 150–220 nm.
Although, as it was mentioned above that the
trifluoromethyl group in a terminal chain often
promotes formation of anticlinic packing, we have not
observed antiferroelectric phase in any of the LC
Fig. 3 Chemical structures of components of non-chiral SmC host and
its phase transitions (temperatures are given in °C).
3
6-38
Synthesis
mixtures containing individual dopant S-FOTDA-6
We suppose that in combination with high helical
twisting power (HTP), very high spontaneous
.
Chiral triflouromethylalkanols (1a, 1b, see Scheme 1)
a
were obtained in a high enantiomeric purity using
4
2,43
polarization is an additional factor for the appearance
of the anticlinic packing because of minimization of the
electrostatic energy excess. Following this hypothesis,
enzymatic kinetic resolution,
that we have modified
4
recently . Synthesis of S-FODTA-6 was carried out as it
4
2
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3
6
was described for its R-analogue from the terphenyl-
dicarboxylic acid chloride ( , Scheme 1, path A).
target S,R-LACTAF-6 and S,S-LACTAF-6
.
A
full
2
description of experimental procedures, data of
materials and instrument specification are given in
Supplementary Information.
However, the diastereomeric compounds S,R- or
S,S-LACTAF-6 cannot be synthesized by path A because
we had failed in obtaining required intermediate
trifluoromethyl lactate 1c or 1d (see Supplementary
Information). Therefore, we have proposed another
strategy where terphenyl core is assembled at the
latest step via cross-coupling of a bromo-benzoic acid
Measurements
The phase transitions temperatures were determined
using polarization microscopy (POM), differential
scanning calorimetry (DSC), dielectric and electro-
optical measurements. Experimental cells were placed
into a specially designed heat chamber; the accuracy of
derivative
Path B).
3 with 1,4-phenyldiboronic acid (Scheme 1,
Briefly, p-bromobenzoic acid was esterified with S-
4
benzyl lactate under carbodiimide methodology to
give chiral ester It’s following catalytic
temperature measurement is
The electro-optical measurements of the tilt angle
(T), deflection angle (E) of the helix principal axis in
the presence of the electric field, response time
.9(T,E) and light transmittance TLT(T,E) were studied in
±0.1°C.
5
3.
θ
Ψ
hydrogenolysis does not occur in ethyl acetate
according to HPLC. Attempts to run the reaction in
methanol lead mainly to transesterification of the ester
τ
0.1-
0
an automatic regime. The basic element of the
experimental setup is computer data acquisition (DAQ)
board NIPCI 6251 from National Instruments. A
3
with methanol and accompanied with the reduction
of aromatic bromine atom (according to GC-MS).
Taking tert-butyl alcohol as solvent allows
a
detailed
description
of
the
electro-optical
suppressing the transesterification however the
reaction mixture still quickly degrades which results in
measurements methods is contained in our previous
36,37,47
works.
a complex mixture where target acid
4 is found by
Measurements of the dependencies TLT(E) and Ψ(E)
allow to evaluate an effective birefringence eff(E) in
HPLC only as one of the major products. Assuming acid-
catalyzed nature of side processes, we have used an
excess of pyridine in order to buffer the reaction
mixture.
ꢀn
48
electric field through the formula, which is valid for
FLCs and AFLCs with sub-wavelength helix pitch:
π
ꢀn (E)d
2
2
eff AFLC
T (E) =sin 2[Ψ(E)+
β
]sin
LT
λ
(1),
where
and
the plane of polarization of the incident light.
The rotational viscosity was measured by an
electro-optic method that is described in details.
Antiferroelectric smectic ferrielectric and
λ
is wavelength, dAFLC is the AFLC layer thickness,
β
is the angle between the rubbing direction and
γ
ϕ
4
9,50
A*
C ,
smectics C* phases were identified by the
characteristic hysteresis loops in polarization P of
planar aligned LC cells on the application of triangular
voltage waveform. The spontaneous polarization for
different LCs has been measured by the polarization
51
reversal current method, which excludes components
of the electric polarization related with the
polarizability of molecules rather than with
supramolecular structure. The dielectric susceptibility
Scheme 1 Synthesis of the diastereomeric S,R- and S,S- LACTAF-6. Reagents
and conditions: i. (S)-benzyl lactate, DCC, DMAP, CH
atm.), Pd/C, pyridine, tert-butyl alcohol; iii. (R)- or (S)-1,1,1-trifluorooctan-2-
ol, DCC, DMAP, CH Cl dppf (cat.),
, 0°C→rt; iv. 1,4-phenyldiboronic acid, PdCl
NaHCO (aq.), SDS, n-butanol, toluene, Δ.
2 2 2
Cl (1
, 0°C→rt; ii. H
G
χ (E) of the AFLC helical structure (known as Goldstone
mode) has been extracted from measured P(E)
2
2
2
3
dependence through the general definition (2):
Thus, target chiral acid
satisfactory yield (65%) and purity (98% according to
HPLC; see Supplementary Information). The acid was
esterified with the corresponding enantiomeric
alcohols 1a and 1b to give diastereomeric esters SR-4
SS-4 The esters SR-4 SS-4 reacted with 1,4-
phenyldiboronic acid by palladium-catalyzed Suzuki
4 was obtained in
1
∂P(E)
T,
χG (E) =
ρ=const
ε0 ∂E
4
(2),
where
ε
0
is permittivity of vacuum, T is temperature,
ρ
,
is the density of a test substance.
.
,
Homeotropic alignment of LC in cells of 20 ꢁ
thickness is assured by coating a thin layer of chromo
m
4
6
cross-coupling reaction in a micro-emulsion to give
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stearylcloride, 5% wt/wt solution in isopropanol, onto
distinguishable only from the plot of the dielectric
susceptibility, (E), versus the electric field, see left
top insertion to Fig. 4. Maximum of the (E)
dependence corresponds to a critical electric field (E
3
6
the substrate. The cells with homogeneous alignment
and thickness 1.5 - 1.7 μm were fabricated with rubbed
nylon-6 layers.
χ
G
χ
G
c
)
of the AFLC helix unwinding, which is a good criterion
for quantitative AFLC characterization in the electric
field.
Results and discussion
2
00
50
00
Both diastereomers of LACTAF-6 and S-FOTDA-6 are
non-mesogenic, however, they are readily soluble in
M1 and even have a stabilizing effect on smectic
phases at low concentrations; the destruction of
smectic ordering is pronounced over 20-25 mol. %
concentration of CD. Nevertheless, due to a wide SmC
temperature range of M1, it is possible to reach CD
concentrations up to 36 mol. % retaining SmC* phase
4
2
1
1
50
0
Ec
0
5
10
15
E (V/ꢁm)
0
ψ
3
0
0.20
ꢀ
neff
20
up to 90
Information).
A preliminary study of ferroelectric properties for
the new CDs of LACTAF series shows that only S,R
diastereomer have the same sign of and helix
handedness as S-FODTA-6 whereas S,S-LACTAF-6
exhibits opposite signs of these parameters. The
doping of M1 with 21 mol % of S,R LACTAF-6 results in
°
C (see phase diagram in Supplementary
-2
0
.15
1
0
0
0.10
0
2
4
6
8 10 12
-4
-
E (V/
ꢁm)
P
s
-15
-10
-5
0
5
10
15
,
E ( V/ꢁm )
-
Fig. 4 (Colour online). Hysteresis of current polarization under applied
an induction of ferroelectric LC phase which is
characterized by the sub-wavelength pitch, and
exceptional hysteresis free electro-optical switching
triangular voltage with frequency f = 1 Hz. Insertion right bottom: deflection
angle
Ψ(E) of the principal axis and effective birefringence ꢀneff(E) at λ=632.8
nm of the AFLC-036 helical structure. Insertion left top: dielectric
susceptibility that corresponds to growing branch of the loop at E>0. All
results have been obtained in 1.7µm electro-optical cell at temperature
with response time ≤50ꢁs. Moreover, we did not
observe any anticlinic phases for the LACTAF based
mixtures. Therefore, because of the favourable opto-
electronic performance of SR-LACTAF-6, we decided to
study it as a second counterpart together with S-
FOTDA-6 in the molar ratios of 4:3 (that is close to their
eutectic) as binary chiral dopants (CDs) for doping in
the host M1. This binary chiral composition is also
readily soluble in M1 without any crucial suppression
of the mesophase for the concentration up to 36 mol.
°
T=24 C.
The transition in the electric field between the
helical and spatially homogeneous structures for a
AFLC occurs near E
c
, where one can also see the abrupt
eff(E) value
approximately 1.7 times) and principle axis deflection
change of the effective birefringence
ꢀ
n
(
angle Ψ(E) of the AFLC helical structure in electric field
right insertion to Fig. 4). Obviously, the saturation
(
%. The most remarkable feature of the four-
level of Ψ(E) corresponds to the tilt angle θ of
molecules in smectic layers.
component mixtures is an unexpected induction of
AFLC ordering for the concentration ≥ 15.0 mol % of
binary CD. Thus, appearance of anticlinic packing for
On the other hand, the
s
P of AFLC-036 decreases
monotonically at
temperature range 0 C
ramp of value within minus 10 C to 0
to melting of the AFLC-036
a higher temperature in the
this mixture (AFLC-015) is observed below 70
for 36 mol % concentrated mixtures (AFLC-036) –
below 76.5 C; the phase sequences are given below:
°C and
°
°
≤T ≤95 C (Fig. 5). The sharp
°
P
s
°C corresponds
°
.
The synthesized CDs possess very high twisting power,
and therefore, the measurement of the pitch by means of
selective reflection becomes a non-trivial problem. The
selective reflection bands, with the decreasing
temperature, move in the UV range, and thereafter,
disappear from the spectrum. Therefore, to estimate the
pitch of the highly doped mixture we are forced to resort
on the electro-optical measurements. Thus in the process,
Anticlinic packing of AFLC-036 in the temperature
range 0 – 77 C unambiguously follows typical P(E)
°
hysteresis loop (Fig. 4), and the saturation levels of the
loop correspond to the spontaneous polarization P
s
2
400 nC/cm . However, in the case of AFLC-
0
first we have measured the helix pitch p value for the
value: Ps ≅
36, in contrast to the typical hysteresis loops of AFLCs,
mixture AFLC-015, i.e. with 15.5% CD concentration, by the
0
selective reflection (insert to Fig. 5).
no pronounced threshold is observed. It can be
4
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6
4
3
1
00
50
00
50
0
500
6c) means that there are no reasonable dark state
15
problems of this AFLC with nanoscale helix pitch.
4
3
2
00
00
00
100
80
Half-pitch
diffraction
100
0
-
-
-
-
150
300
450
600
60
Full pitch
diffraction
4
00
600 800
Wavelength, nm
1000
-100
0
20
40
T ( C)
60
80
100
o
s
Fig. 5. Temperature dependence of the spontaneous polarization P of
the AFLC-036 (open circles) measured at heating from preliminary
°
obtained solid crystal state starting from -10 C. The temperature
dependence of the pitch for AFLC-015 is represented by the black
o
circles. The solid line shows extrapolation of the measured p values to
the low-temperature region. The insertion shows the selective light
diffraction for the mixture of AFLC-015 of binary chiral dopant in M1
host at oblique light incidence ( = 70 ) taken at 78.1 C as an example.
ξ
°
°
The temperature dependence of the pitch for this
mixture has been shown in Fig. 5, at the room
temperature the pitch is around 140nm. Both the full
and the half pitch peaks were observed for the inclined
5
2-54
cell, which is also described the previously.
On the
other hand, the electro-optical relaxation time (
τoff) for
Fig. 6. (a) Temperature dependencies of the tilt angle (open circles) and
ϕ
rotational viscosity γ (black circles) of the AFLC-036 are presented; (b) and
(c) are textures of planar aligned 1.7µm AFLC-015 and AFLC-036 layers
small deformation of the helix can be given by
2
off ϕ ϕ 0
τ ≅ γ /K q
placed between crossed polarizers, respectively. The size of the images is
(
3).
2
00×250µm, the voltage is not applied.
Here K
is the wave vector of the helix. Thus, by comparing
the two off according to (3) for the two mixtures with
known , we can estimate the value of the pitch for
the highly concentrated mixtures. In ref. 55, it is
experimentally established that the K is almost
independent on the CD concentration. Thus, we
assume that the change in the K for the AFLC-015 and
AFLC-036, is insignificant. Whereas, as expected, the
decreases at higher temperatures. At the temperature
of 40 C, the of the AFLC-036 is~0.2 Pa·s (Fig. 6a),
however, for the AFLC-015, in the same ambient and
experimental conditions, the ~0.095 Pa·s, the smaller
CD concentration in AFLC-015 is responsible for the
lower viscosity. Furthermore, the OFF of the AFLC-036
and AFLC-015 are 20 and 150µs respectively. Thus, on
comparing the pitch of AFLC-015 at 40 C (p =160nm,
ϕ
is the elastic constant of the helix twisting and
Additionally, the alignment optical quality is better
q
0
for the AFLC-036, in other words, the density of spatial
inhomogeneities of the AFLC layer decreases for the
highly concentrated AFLC, see Figs 6 (b) and 6 (c),
which compare the optical quality of the AFLC 015 and
τ
γ
ϕ
ϕ
AFLC 036. The smaller p of the AFLC-036 is probably
0
responsible for the better alignment quality.
ϕ
Behaviour of the two parameters: (i) the principal
optical axis of the helical structure (right insertion to
Fig. 4) which deflects continuously and almost linearly
γ
ϕ
°
γ
ϕ
by an angle
χG when E
the well-known electro-optical DHF-effect in smectic-
Ψ
(E) up to 10 – 11° and (ii) almost constant
≤
0.8E (left insertion to Fig. 4), are similar to
c
γ
ϕ
5
6-58
C* helical structure.
Additional evidences for the similarity of electro-
optical behaviour of AFLC cells for E Ec and the DHF-
τ
≤
°
0
effect in smectic C* phase are given in Fig. 7 wherein,
the characteristic relaxation times increase sharply in
see Fig.5 for a clarification) with the pitch of AFLC-036,
in the light of eq. (3), the pitch for the AFLC-036 turns
the close vicinity of E , but almost independent on
c
out to be ~36nm at 40
Additionally, the AFLC-036 is characterized by a
weak temperature dependence of the tilt angle (T) for
the temperature below 60 C (Fig. 6a), which is
favourable for applications in various display and
photonic devices. Furthermore, satisfactory
°
C.
applied electric field at E << Ec.
Thus, form the considerations based on above-
mentioned facts it can be affirmed that the electro-
θ
°
optical behavior of the AFLC cells, for E < E , is similar to
c
the deformed helix ferroelectric liquid crystals, and
therefore, we can term it as the deformed helix
antiferroelectric (DHAFLC) effect.
a
alignment quality of the AFLC-036 (see insertion to Fig.
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The applied stair waveform of the applied electric
field E (see the inset Fig. 7) allows to measure the
switching ON ( on) and switching OFF ( off) times,
separately. In contrast to electro-optical effects in
nematic liquid crystals (NLCs), where usually
in the range of few milliseconds at the best, DHAFLC
effect exhibits off with at least 1-2 orders of
magnitude faster than that of NLCs. The values of the
In addition, the contrast ratio (СR = TLTmax/TLTmin)
reaches 1000:1 with DHAFLC electro-optical effect (see
insets of Fig. 7 and Fig. 8) that provides promising
prerequisites for applications because of fast and
stable operations in a broad temperature range.
However, the light transmittance TLT is limited to 40%
and further research is required to improve it for the
modern applications.
τ
τ
τon << τ
off
τon >> τ
switching times are
).
τ
on
≅
400 µs, and
τ
off
≅
20 µs (Fig.
c
At E > E the light transmittance of cells jumps up
7
and tends to the saturation very fast, forming a typical
electro-optical AFLC hysteresis loops at repolarization
of the applied voltage, and the shape of the loops is
highly dependent on the shape and frequency of the
applied voltage, Fig. 9. As it is well-known fact that the
characteristics of the deformed helix mode strongly
2500
2000
1500
1000
on
τ
1
00
T
E
10
τoff
10
1
0
10
-20
30
-
47,59
0
,1
,01
E-3
E-4
depend on the frequency of the applied voltage,
0
1
1
-
therefore, we have studied the dispersion parameters
of hysteresis loop that is plotted in Fig. 9. Interestingly,
DHAFLC
region
0
10 20 30
t (ms)
the voltage coercivity measured at applied triangular
triang
voltage (V
c
) is always larger than that at applied
stair
500
stair voltage (V
triang
c
stair
), and frequency dispersions
V
c
(f) and V
c
(f) are quite opposite Fig. 9(b).
triang
0
Moreover, the profile of V
c
(f) dependency is exactly
4
Ec
8
12
in the same as in the case of smectic C* phase because
5
of viscous torque.
9,60
E (V/ꢁm)
Fig. 7. (Color online). Dependencies of switching on (τon) and off (τoff) times of
AFLC-036 electro-optical response on applied electric field at temperature T
stair voltage from - to +
stair voltage from + to -
°
=
1
30 C,
β = 0, wavelength λ = 632.8 nm, and the applied voltage frequency f =
00 Hz. The inset shows the electro-optical response of LC cell (the lower
0,8
0,8
curve) from applied stair voltage.
0,6
0,6
triang
Vc
stair
Vc
0,4
0,4
At β = Ψ the electro-optical response time τ0.1-0.9
measured at applied square voltage and the TLT value
both are almost independent of temperature in a range
0,2
0,2
(a)
0°C – 45
0,0
0,0
2
°
C in DHAFLC mode, herewith TLT
65 - 70µs under conditions
mentioned above (Fig. 8).
≈ 40%.
-20
-10
0
10
20
Note, that τ0.1-0.9
≅
V (Volt)
100
10
1
Transmittance TLT
stair
Vc
4
0
3
00
triang
Vc
100
1
0
E
10
0
250
200
150
30
1
-10
20
30
40
50
60
-
(b)
0
,1
,01
,001
E-4
-
0,1
1000
0
-
1
10
100
0
20
-
f (Hz)
1
T
LT
-
0
1000
2000
3000
Fig. 9 (Color online): (a) AFLC-036 electro-optical hysteresis loops measured
100
t (
ꢁ
s)
at continuous repolarization of triangular and stair applied voltage at T =
10
5
0°C,
voltage frequency f = 5 Hz; (b) dispersion curves V
β = 0, λ = 632.8 nm, the AFLC layer thickness 1.5 µm, and the applied
50
triang
stair
Response time τ0,1-0,9
(f) and V
(f)
c
c
measured at the same settings as given in the caption to Fig. 8 (a).
0
0
20
25
30
35
40
45
o
stair
T ( C)
Whereas for V
c
(f) reduces almost to zero as the
frequency increases, see Fig. 9(b). Therefore, we
designed a new driving signal with two frequency and
magnitude that is capable of offering a hysteresis-free
U-shaped electro-optical response (see Fig. 10), with
simple adjusting the frequency of driving stair voltage
Fig. 8. (Color online). Temperature dependencies in DHAFLC mode of
the electro-optical response time 0.1-0.9 and light transmittance
compared with the transmittance of an empty cell placed between
crossed polarizers) of 1.5µm thick AFLC-036 layer placed between
crossed polarizers at = 1 kHz, = 632.8 nm. Applied square
electric field tension is µm, see insert to Fig.8.
τ
T
LT
(
β
E
=
Ψ
,
f
λ
= 10V/
6
| J. Name., 2012, 00, 1-3
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ARTICLE
to f > 100 Hz, which is followed by Fig. 9(b). To the best
our knowledge, this is first experimental observation of
the effect, which was predicted by Petrenko and J.W.
2
9
Goodby in 2007.
The region from 0 to ± threshold voltage at the
plots of both hysteresis and hysteresis-free EOP
response often referred to pre-transition or pre-
threshold phenomena. Their physical essence is the
DHAFLC mode, which is a first stage of the U-shape
switching and its contribution to the light
transmittance strongly depends on the waveform, the
frequency of applied voltage (as seen from the
comparison chart Fig. 9(a) and Fig. 10), and on the Ψ
angle. Thus, it is possible to choose either DHAFLC or
U-shape effects by simple changes in the driving
settings.
Fig. 11 Evolution of AFLC-015 electro-optical hysteresis loops dependently on
T at 337 Hz from +20V to -20V
T at 337 Hz from -20V to +20V
the helix pitch. Measurements have been carried out at continuous
°
8
6
4
2
0
0
0
0
0
1,8
20
0
repolarization of stair applied voltage at
β
= 0, λ = 632.8 nm, and (a) T = 65 C,
°
°
p
0
= 230nm; (b) T = 40 C, p
0
= 160nm; (c) T = 24 C, p
0
= 140nm.
1,2
-
20
Conclusions
-40
0,6
In summary, use of two types of chiral dopants, i.e.
esters of terphenyldicarboxylic acid containing in side
-60
0,0
0
30
60
90
wings
chiral
2-trifluoromethylheptanol
or
t (ms)
trifluoromethylheptyllactate, induces unusual AFLC
mesophase with nanoscale helix pitch in achiral smectic
C host, biphenyl-pyrimidines. Two fast electro-optic
effects in this mixture have been observed, first,
DHAFLC and second, hysteresis free U-shape switching,
which evolves from the V-Shape as the pitch decreases
DHAFLC mode
-20
-10
0
10
20
V (Volt)
Fig. 10 (Color online). Hysteresis free U-shaped electro-optical response
°
under applied stair voltage at T = 60 C,
β = 0, λ = 632.8 nm, the AFLC-036
(see fig. 11).
layer thickness is 1.5 µm, and the applied voltage frequency f = 337 Hz. Insert
shows electro-optical response of the cell (at the bottom) at applied dual
frequency voltage (atop).
The DHAFLC provides fast, almost temperature
independent switching time (< 65 µs see Fig. 8) with
the contrast ratio ~1000:1. The good alignment quality
of ultra-short helix pitch (< 100 nm) manifests such a
high contrast ratio.
Furthermore, the said U-shape switching strongly
depends on the pitch of the AFLCs. Plots for the U-
shape switching at different pitch length, at different
temperatures, have been plotted in Fig. 11. It is clear
from the figure that the width and the threshold
voltage increase at lower pitches. The dashed lines
represent the threshold voltage for the AFLC with pitch
The U-shaped electro-optical response of the
proposed AFLCs is similar to NLCs, in shape, but two
orders of magnitude faster. Both are hysteresis free,
show a threshold, and therefore, the proposed AFLCs
can be driven by the same driving waveforms as
conventional LCDs and photonic dual frequency voltage
6
1
~
140 nm that shows a clear shift at higher pitches. In
devices.
other words, we can say that the threshold voltage
The DHAFLC mode, in comparison to the U-shape
mode, offers higher contrast ratio that can be
attributed to the optical axis (OA) modulation around
the easy axis. For the dark state, the OA in the DHAFLC
mode is parallel to the polarization azimuth of the
impinging light, and therefore, the impinging light feels
minimum retardation that results in a dark state with
smaller transmittance and higher contrast ratio. On the
other hand, U-shape AFLC mode reveals insufficient
dark state, because it is a result of averaging of the OA
decreases for the larger pitch. The threshold voltage is
2
related with the E
c
, which is E
c
∝
q
0
. The q
o
decreases
and
threshold voltage follow the same trend at the higher
with the increase in pitch, and therefore, the E
c
9
,24
pitch.
Hence, we can conclude that the V-shape
switching of the AFLC with relatively larger pitch
transforms to U-shape switching at smaller pitches.
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ARTICLE
Journal Name
deflection in the DHAFLC mode (see fig. 10), and
therefore, it provides less contrast ratio (around 150:1)
in comparison with the DHAFLC.
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(
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