Optically isotropic state in bent core nematic mixtures with rod like molecules inducedby direct current electric fieldOmaima Elamain, Gurumurthy Hegde, Katalin Fodor-Csorba, and Lachezar Komitov Citation: Applied Physics Letters 103, 163501 (2013); doi: 10.1063/1.4824974 View online: http://dx.doi.org/10.1063/1.4824974 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/103/16?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Electric field induced biaxiality and the electro-optic effect in a bent-core nematic liquid crystal Appl. Phys. Lett. 96, 011106 (2010); 10.1063/1.3280817 Direct confirmation of biaxiality in a bent-core mesogen through the measurement of electro-optic characteristics J. Appl. Phys. 105, 094509 (2009); 10.1063/1.3108486 Comment on “Dynamics of electro-optical switching processes in surface stabilized biaxial nematic phase foundin bent-core liquid crystal” [J. Appl. Phys.101, 034105 (2007)] J. Appl. Phys. 104, 036104 (2008); 10.1063/1.2963702 Electro-optic characteristics of 90° twisted nematic liquid crystal display driven by fringe-electric field J. Appl. Phys. 95, 1625 (2004); 10.1063/1.1636253 Time-resolved optical waveguide study of the reorientation in a nematic liquid crystal under applied electric field J. Appl. Phys. 81, 1135 (1997); 10.1063/1.363859

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Optically isotropic state in bent core nematic mixtures with rod likemolecules induced by direct current electric field

Omaima Elamain,1 Gurumurthy Hegde,2 Katalin Fodor-Csorba,3 and Lachezar Komitov1,a)

1Department of Physics, University of Gothenburg, SE-412 96 Gothenburg, Sweden2Faculty of Industrial Sciences and Technology, UMP, 26300 Gambang, Kuantan, Pahang, Malaysia3Research institute for Solid State and Optics of the Hungarian Academy of Science H-1525, Budapest, Hungary

(Received 3 August 2013; accepted 29 September 2013; published online 14 October 2013)

One of the most important characteristic of the liquid crystal displays is the contrast of the

generated images. The dark state of the display is a prerequisite for a high image contrast.

Optically isotropic state was observed in bent core nematic mixtures containing rod like molecules

induced by low applied dc electric field. It is found that the switching properties of these nematic

mixtures depend on the concentration of the rod like molecules. Comparatively high concentrations

of rod like molecules in the mixtures resulted in an improvement of the contrast and switching

properties of the bent core nematic mixtures, thus demonstrating their potential for display

applications. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4824974]

The operation of the majority of the conventional liquid

crystal displays (LCDs) is based on the re-orientation of the

liquid crystal (LC) molecules from field-off to field-on posi-

tion driven by the coupling between dielectric anisotropy of

the LC material and an applied electric field (dielectric cou-

pling). Such a molecular re-orientation by the applied field,

however, results in an electro-optic effect due to anisotropic

optical properties of the LCs. Depending on the sign of the

LC dielectric anisotropy, the electric field in LCDs is applied

along the preferred direction of LC alignment or perpendicu-

lar to it, if this sign is positive or negative, respectively.

Generally, there are two conventional ways of applying an

electric field in the LCDs, across the cell gap (out-of plane)

or along the confining substrates (in-plane). Conventional

LCDs contain a slab of nematic liquid crystal with rod shape

molecules (calamitic LCs). This class of liquid crystals is

known to be optically uniaxial, i.e., they exhibit birefrin-

gence. Recently, the discovery of nematic LCs constituting

of bent core (BC) molecules draw the attention of scientists

and engineers due to the predicted biaxial properties which

they may have and the advantages which these LC materials

may offer for application in displays as fast switching, for

instance.1–3 However, the biaxiality of BC nematics is still

an issue under discussion.

We recently reported on field-induced optically isotropic

state in a BC nematic, which was considered as unambiguous

proof of the biaxial optical properties of this material under an

applied dc electric field.4 It was argued that the applied dc

electric field results in a bias of the rotation of BC molecules

around their long molecular axis, due to the coupling between

the applied field and the net molecular dipole moment (polar

coupling) of the BC molecules. Such a bias, however, resulted

in alignment of the arms of BC molecules in a plane making

an angle with the confining substrates of the sample which

increased when the field was increased. The alignment of the

BC molecules in a plane, so called molecular plane, gave rise

to biaxial optical properties of the BC nematic. These

properties were compared with those of the antiferroelectric

liquid crystal (AFLC) with large molecular tilt, which became

close to or equal to 45� (AFLC 45�) at certain temperature.

When the AFLC 45� is aligned in bookshelf geometry, i.e.,

with smectic layers normal to the cell substrates and mole-

cules lying in the substrate plane, then the AFLC 45� exhibits

optically isotropic state (with Dn¼ 0). Likewise AFLC 45�,the BC nematic was found to exhibit an optically isotropic

state at certain magnitude of the applied dc electric field, i.e.,

at certain tilt of the molecular plane of the BC molecules with

respect to the display cell substrates. In pure BC nematic, the

applied dc electric field induced optically isotropic state (with

Dn¼ 0) from the field-off birefringent state (with Dn 6¼ 0).4

On the contrary, in the case of AFLC 45� as well as in the

case of Blue Phases (BPs), the applied electric field resulted in

a transition from field-off optically isotropic state to field-on

optically birefringent state.5,6

In this work we report on the electro-optical behavior of

mixtures of a pure BC nematic with rod like (RL) mesogens,

so called BC/RL nematic mixtures. The main goal of this

study was to find the upper limit of the concentration of RL

mesogen in BC/RL binary nematic mixture, which still allow

the generation of optically isotropic state in the sample by

the applied dc electric field.

The molecular structure and the phase transition tempera-

tures of the host BC nematic liquid crystal material, we studied

in the present work, are shown on Fig. 1. The synthesis of the

material and its phase sequence are reported in Ref. 7. This

BC liquid crystal material exhibits a temperature interval of

the nematic phase 64<N< 75 �C. As a guest material with

RL molecules, the commercial nematic mixture MLC6608

(Merck) was chosen. Both, host and guest, nematic liquid crys-

tals are with negative dielectric anisotropy (De < 0). Hence,

their BC/RL mixtures are also expected to be with De < 0.

Usually, BC liquid crystal materials have nematic phase

at high temperatures and within the narrow temperatures

interval (in some cases it is a couple of degrees). One possi-

ble approach to reduce the temperature of the nematic phase

and to broaden its temperature interval, thus making these

materials attractive for device applications, is to mix pure

a)Author to whom correspondence should be addressed. Electronic mail:

komitov@physics.gu.se

0003-6951/2013/103(16)/163501/4/$30.00 VC 2013 AIP Publishing LLC103, 163501-1

APPLIED PHYSICS LETTERS 103, 163501 (2013)

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BC nematic material with RL mesogenic molecules. The

concentration of RL mesogenic guest in the BC/RL nematic

mixtures studied in this work was varied in the range of

0–50 wt. %. It was found that the temperature range of the

nematic phase of these mixtures increased with the concen-

tration of the RL mesogenic guest molecules in the BC/RL

nematic mixtures (see Table I).

Experimental cells of conventional sandwich type, with

a cell gap of about 2 lm (homemade as well as EHC, Japan),

were used in our experiments. The cell substrates inner sur-

face was pre-coated with transparent ITO electrodes on top

of which polyimide alignment layer promoting planar align-

ment (PA) was deposited and unidirectionally rubbed in

order to obtain uniform PA with preferred direction along

the rubbing direction.

The pure BC nematic liquid crystal and its mixtures

with RL mesogens were filled into the experimental cells in

isotropic phase by means of capillary forces. The quality of

the PA of the cells was inspected by polarizing microscope

with crossed polarizers. The position of the cell slow axis

(i.e., the sample slow axis) was detected by inserting a k-red

optical plate between the sample and the analyzer of the

polarizing microscope with its slow axis oriented at 45� with

respect to the transmission direction of the analyzer.

All the experimental cells exhibited uniform PA with

cell slow axis oriented in the field-off state along the rubbing

direction of the alignment layer in the cells.

The electro-optic response of the cells was investigated

by means of optical polarizing microscope with crossed

polarizers. It was detected by a photodiode connected to

Tektronix TDS 540 digital storage oscilloscope.

We have chosen in this study pure BC nematic, as a

host material, and RL nematic, as guest material, both with

De< 0, in order to obtain binary BC/RL nematic mixtures,

containing different concentrations of the RL mesogen, also

being with De < 0. We have chosen also surface treatment of

the inner cell substrates which promotes PA in the experi-

mental cells in order to ensure that the coupling between the

dielectric anisotropy of the nematic material and the applied

dc electric field (dielectric coupling) will only stabilized the

PA in the experimental cells but will not give rise to any

electro-optic response. Hence, if any electro-optic response

will occur in the cells, the dielectric coupling will be com-

pletely excluded as a possible origin of this response.4

What we have observed experimentally was that the dc

field applied across the experimental cell was re-orienting

the pure BC material as well as its BC/RL nematic mixtures,

with concentrations of the RL guest equal or below 40 wt. %,

away from the PA thus giving rise to a very attractive

electro-optic effect, field-induced optically isotropic state

with Dn¼ 0.

As well known, the quality of the dark state in LCDs is

one of the most important characteristics of the LCDs. The

highest quality of the dark state in the LCD, inserted between

two crossed polarizers, is obtained when in this state the LC

slab exhibits optically isotropic properties, i.e., it possesses

Dn¼ 0.

Switching of the cell containing 30 wt. % BC/LC ne-

matic mixture under an applied dc electric field at different

voltages is depicted in Fig. 2. The cell is oriented with its

slow axis (coinciding with the rubbing direction of the align-

ment layer) at 45� with respect to the transmission direction

FIG. 1. The components of the BC/RL nematic mixtures studied in this

work. The chemical structure of the BC liquid crystal and the phase transi-

tion temperatures of BC and RL nematic components.

TABLE I. Contrast ratio and nematic phase interval of BC nematic and its

BC/RL nematic mixtures as a function of the RL concentration.

Concentration of RL (wt. %) 0 7 20 30 40

Temperature interval of nematic phase (�C) 11 22 34 40 47

Contrast ratio 770 850 918 1060 1055

FIG. 2. Cell, with gap of about 2 lm,

containing a slab of 30% BC/RL ne-

matic mixture inserted between two

crossed polarizers, with rubbing direc-

tion at 45� with respect to the transmis-

sion direction of the polarizers, at

different dc voltages: (a) V¼ 0, (b)

V¼ 5 V, (c) V¼ 6 V, and (d) V¼ 10 V.

The rubbing direction and the cell slow

axis are indicated by arrows assigned as

n and s.a., respectively.

163501-2 Elamain et al. Appl. Phys. Lett. 103, 163501 (2013)

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of the crossed polarizers. In this arrangement, the cell in

field-off state is in its bright state (c.f. Fig. 2(a)). At lower

voltages than 6 V, the birefringence of the cell is continu-

ously decreasing with the applied voltage; thus, a grayish

state is generated in the cell (Fig. 2(b)). The optically iso-

tropic (dark) state in this cell has been obtained at about 6 V

(Fig. 2(c)). The dark state does not change upon rotation of

the cell between the crossed polarizers, thus confirming that

the field-induced state possesses Dn¼ 0 (see Fig. 3). At

applied voltages higher than 6 V, the birefringence of the

cell start to increase with the voltage thus a low birefringent

state starts to merge at higher voltages (c.f. Fig. 2(d)).

Likewise the pure BC material investigated previously,4 the

position of the slow axis of the cells containing the BC/RL

nematic mixtures with concentration of the RL guest up to

40 wt. % was found to reorients gradually, under a dc electric

field applied across the cell gap, from position parallel to the

preferred direction of alignment, at field-off state, to position

perpendicular to it and to the cell substrates, at field-induced

optically isotropic state (dark state), and to flip, at higher

applied voltages, to position parallel to the substrate and per-

pendicular to the field-off state position of the slow axis.

Hence, the slow axis in these three states of the cell has

mutually orthogonal directions, as depicted in Fig. 2.

However, neither field-induced optically isotropic state

nor field-induced switching of the cell slow axis between the

mentioned above three mutually perpendicular directions

were found to take place for concentration of the RL meso-

genic guest in these particular BC/RL nematic mixtures con-

taining RL mesogenic host with concentration above

40 wt. %. This means that the polar coupling giving rise to

field-induced optically isotropic state in the BC/RL nematic

mixtures was completely suppressed for the concentrations

of the RL guest higher than 40 wt. %. This is because the PA

of the BC/RL nematic mixture was stabilized completely by

the dielectric coupling and no more any re-orientation of the

mixtures molecules took place because the dielectric cou-

pling was dominating over the polar coupling.

The voltage Vth required to induce optically isotropic

state in the BC/RL mixtures was found to depend on the

concentration of the RL mesogenic guest (c.f. Fig. 4). It

was slightly increased when this concentration increased.

Moreover, the measured contrast of the cells with different

concentrations of RL mesogen seems to increase with the

concentration of the RL mesogen. This observations indi-

cates that RL mesogenic guest with Deffi 0 might be more

appropriate as a guest RL mesogenic since it may allow to

increase the concentration of the RL guest without affec-

ting the magnitude of Vth and thus to improve the

performance of the BC/RL nematic mixtures. Such study is

underway.

It should be noted here that the rotational viscosity of

pure BC nematic materials is reported to be almost one order

higher than the one of calamitic (RL) nematics.8–11 Hence,

longer switching times are expected for BC nematic liquid

crystals. Indeed, the switching times for the pure BC nematic

material studied in this work was measured to be: rise time

sr¼ 400 ms and fall time sf¼ 300 ms. However, even small

concentrations of the RL mesogenic guest (of about 7 wt. %)

in the BC/RL nematic mixtures, reduced the response times

about 4–5 times. The fall time, which is directly proportional

to the rotational viscosity and it does not depend on the

applied field, was reduced more than one order of magnitude

in the BC/RL nematic mixture containing RL mesogen of

about 40 wt. %. These observations indicated that the dynam-

ical characteristics of the BC/RL nematic mixtures could be

tailored by a proper choice of their components and relative

concentrations so that BC/RL nematic liquid crystal mixtures

with desired characteristics could be prepared. By proper

choice and relative concentration of the pure BC nematic

and the RL mesogen in the binary BC/RL nematic mixture

resulting a nematic liquid crystal mixture with Deffi 0, which

the characteristics may essentially be improved. This demon-

strating the usefulness of such an approach for preparation of

BC/RL mixtures appropriate for device applications.

Probably, multi-component nematic mixtures containing BC

nematic component could be a promising approach in

FIG. 3. Transmission light intensity ver-

sus the applied voltage for the cells con-

taining pure BC nematic material (c.f.

Fig. 1) and its mixtures with RL meso-

genic guest at different concentrations.

FIG. 4. Cell from Fig. 2 under dc voltage V¼ 6 V, rotated from field-off

position of the rubbing direction of the cell at 45� (a) to position 0� (b), with

respect to the transmission direction of the polarizers. The bright line in the

middle of Fig. 4 is the edge of the electrode.

163501-3 Elamain et al. Appl. Phys. Lett. 103, 163501 (2013)

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searching nematic mixtures exhibiting field-induced opti-

cally isotropic state with short switching times and ability to

generate switching of the sample slow axis between three

mutually orthogonal positions.

Importantly, field-induced optically isotropic state in the

binary BC/LC nematic mixtures seems to be a very attractive

alternative of the field-induced birefringence in optically iso-

tropic Blue Phases, since it requires one order lower voltages.

The referee’s comments and suggestions are highly

acknowledged.

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