Japanese Journal of Applied Physics
Vol. 45, No. 37, 2006, pp. L1013–L1015#2006 The Japan Society of Applied Physics

Characterization of Nematic Phase of Banana Liquid Crystal
Khoa Van LE, Surajit D HARA1 , Bukkinakere Kapanipathaiya SADASHIVA2 ,Yoichi T AKANISHI and Hideo T AKEZOE
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan
1
School of Physics, University of Hyderabad, Hyderabad-500046, India
2Raman Research Institute, C. V. Raman Avenue, Sadashivanagar, Bangalore-560080, India(Received July 1, 2006; accepted August 7, 2006; published online September 15, 2006)

We report the ﬁrst detailed optical and electrical characterizations of the nematic (N) phase of a banana liquid crystal, i.e.,
pretilt angle, and optical and dielectric anisotropies, and point out the diﬀerence from those of rodlike molecules. The pretilt
angle measurements using a conventional crystal rotation method show that the pretilt angle is very small (less than 1 ) evenin cells with a polyimide ﬁlm yielding a high pretilt angle like 8 for calamitic molecules. We also measured the birefringencen in the N phase as a function of temperature and estimated the orientational order parameter S. The medium shows a small
negative dielectric anisotropy in the N phase and the anisotropy becomes large in the B2 phase mainly because of the tilt of themolecules from the layer normal. [DOI: 10.1143/JJAP.45.L1013]
KEYWORDS: banana liquid crystal, nematic phase, pretilt angle, birefringence, dielectric constant, order parameter

Liquid crystal is a soft matter and the soft nature ofthe medium couples with anisotropic optical and dielectricproperties giving rise to many electrooptic eﬀects at
relatively low voltages. These are exploited in liquid crystal
displays (LCDs), which are the lowest-power-consuming ﬂat
panel devices. The tremendously fast development in LCDs
is due to the progress in the physics and chemistry of liquid
crystals during the past three decades. Most of the display
devices make use of nematic (N) liquid crystals made of
rodlike molecules. The discovery of electrooptical switchingin banana liquid crystals has also created enormous interestin this ﬁeld.1,2) Recently, a new liquid crystal display mode
has been demonstrated by Shimbo et al.,3) which appears tobe promising for future display applications owing to its
various advantages over all existing modes. For suitableapplications, the characterization of materials is essential.
So far, there are not many reports about the determination of
the physical properties in terms of, for example, birefringence, elastic moduli, anchoring energy, and pretilt angle, of
these materials. Measurements of these physical parameters
are important from the fundamental as well as the technological point of view. The main diﬃculty in measuring suchphysical parameters in banana liquid crystal is the pooralignment. Recently, Reddy et al. have reported theobservation of the N phase in a homologous series ofbanana liquid crystal.4) The director can be well aligned ina cell treated for homogeneous alignment. In this letter, we
report the measurement of pretilt angle, birefringence anddielectric constants in the N phase of a banana liquid crystal.
The chemical structure of the compound is shown inFig. 1. The details of the synthetic scheme were reportedearlier.4) The compound exhibits the phase sequence Cry99.6  C-B2-114.5  C-N-119.5  C-Iso as observed using apolarizing microscope upon cooling. Glass plates were spincoated with polyimide (Nissan, SE 7492) and cured at
180  C for 30 min for homogeneous alignment. After curing,
the plates were rubbed and assembled such that the rubbing
directions were antiparallel. The cells were ﬁlled in theisotropic phase and cooled slowly to the N phase, resulting
in a good homogeneous alignment of the director. For
homeotropic alignment, glass plates were spin-coated withpolyimide (JSR, JALS 204) and cured under the sameO

H3COOOOOOHCH3OFFH

C14H29O

OC14H29Fig. 1. Chemical structure of compound used.

conditions. The temperature of the sample was controlledusing a temperature controller within an accuracy of 0.1  C.
Texture observation was carried out using a polarizing
microscope. All the measurements were made while coolingthe sample from the isotropic phase.
Pretilt angle is deﬁned as the angle of the average opticalaxis of the molecules relative to the substrate surface. Weemployed a conventional crystal rotation method (CRM) for
measuring pretilt angle,5) which is generally used foruniaxial N liquid crystals made of rodlike molecules. A
He–Ne laser (632.8 nm) was used to illuminate a 40-mmthick cell and transmitted intensity was measured as afunction of incidence angle from 45 to +45 about the axis
perpendicular to the light beam and rubbing direction. The
transmitted intensity was ﬁtted to a theoretical equation,5,6)
and the value of pretilt angle was obtained as a ﬁtting
parameter.
The variation in the birefringence of the sample at  ¼
550 nm was measured using a polarizing spectrophotometer(ORC, TFM-120AFT-PC) as a function of temperature inthe N phase. The transmittance spectra were ﬁtted to a wellknown equation for nonabsorbing birefringent media todetermine n, where the dispersion of the refractive indices
was properly taken into account. In this measurement, thecell thickness was 5.4 mm. The dielectric measurements wereconducted with an impedance analyzer (Yokogawa-HewlettPackard, 4194A) using homogeneous ("? ) and homeotropic("k ) cells.
The textures of the N phase under cross polarizers areshown in Fig. 2. In a homogeneous cell, it was completelydark (a) when the optic axis was parallel to either of the twopolarizers and gradually turned to be bright (b) when thesample was rotated. Complete darkness indicates that theL1013

K. V. LE et al.

(b)

(a)

PP
A

A

(c)

3Pretilt angle (deg)Transmittance (arb. units)

Jpn. J. Appl. Phys., Vol. 45, No. 37 (2006)

210-20(d)0114

115-10116
117
118
Temperature (°C)

01011920Incident angle (deg)PA

1Fig. 3. Incidence angle (inside cell) dependence of transmitted intensity
for 40-mm-thick nematic layer at 116  C. The pretilt angle estimated by
the best ﬁt is 0.30 . There is no temperature dependence, as shown in theinset.

P
A

Fig. 2. Texture of homogeneously aligned 5.4-mm-thick sample at 118  C;
nematic phase under crossed polarizers with director parallel to either
polarizer or analyzer direction (a) and rotated 10 from the (a) position(b). Texture of homeotropically aligned 5.7-mm-thick sample at 119  C(N) (c) and at 111  C (B2) (d).

Birefringence

0.10
0.09
0.08
012
3
TNI -T (°C)

45Fig. 4. Variation of birefringence with temperature.

Order parameter

major molecular axis aligns well along the rubbing direction.
On the contrary, the brightness in Fig. 2(b) is not perfectly
uniform. Because of the bent core of the molecule used, thefree rotation of individual molecules is prohibited to someextent, resulting in birefringence. Hence, macroscopic
biaxiality emerges and the rubbing treatment cannot alignthe biaxial axes. In a homeotropic cell, however, a completely dark view was not observed in the N phase (119  C)(c). The absence of complete darkness is attributed to themolecular biaxiality and the resultant macroscopic biaxial
order, as suggested by Fig. 2(b). When a layer structure wasformed in the B2 phase (111  C) (d), schlieren texture wasvisible owing to the biaxiality. In addition to the emergenceof biaxiality, layer deformation is associated with the N–B2
phase transition. This explains why the darkness in the B2phase is less pronounced than that in the N phase.
A representative variation in transmittance intensity in theCRM at 116  C is shown in Fig. 3. The circles represent theexperimental data points and the solid line denotes the best
ﬁt based on the theoretical equation. The pretilt angles
measured at four diﬀerent temperatures are plotted in theinset of Fig. 3. It is found that the pretilt angle of banana
liquid crystals is extremely small with the alignment layer(Nissan, SE 7492), which generally yields large pretilt angle
for calamitic liquid crystals. On the basis of our measurements using the same alignment layer and the same rubbing
strength, we obtained about 8 in 5CB. Such a small pretilt
angle may originate from the presence of long ﬂexible alkyl
chains on both sides of the bent core and/or from atransverse dipole. A systematic study must be performed toelucidate the real cause of the small pretilt angle.
We also measured the birefringence in the N phase as afunction of temperature. The variation in birefringence isshown in Fig. 4. It is noticed that birefringence increases
abruptly at the isotropic–N phase transition, indicating a ﬁrst
order phase transition, and gradually increases as temperature decreases in the N phase. The temperature dependenceof n can be approximated for N liquid crystals by theformula n ¼ n0 ð1  T=T1 Þ , where T1 and  are theadjustable parameters and n0 is the birefringence of the0.11

0.9
0.6
0.3
0.001234
TNI -T (°C)

5Fig. 5. Variation of order parameter with temperature.

perfectly aligned sample. The equation ﬁts well in the entire
temperature range, as shown in Fig. 4. By least squares
ﬁtting, we obtained  ¼ 0:12 and n0 ¼ 0:18. Similarvalues of  are also reported in several other nematics.7)
The order parameter S was estimated using the relation S ¼
n=n0 . The temperature variation of the calculated orderparameter is shown in Fig. 5. The order parameter in the Nphase just below the phase transition was approximately 0.4
and increased to 0.6 before the N–B2 transition took place.
The temperature dependence of S was also measured byWeissﬂog et al.8) and Gong et al.9) in the nematic phase of
a banana liquid crystal by nuclear magnetic resonancespectroscopy and a similar temperature dependence withslightly smaller values of S was reported.
Finally, we show the results of the dielectric measurements in Fig. 6. In the N phase, a small negative dielectricanisotropy was obtained. The negative anisotropy originatesfrom carbonyls and substituted F atoms. When the phasetransition from N to B2 takes place, "? increases, whereas "kdecreases. The increase in "? originates from the cooperative response of molecular dipoles due to molecular close

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Real part of dielectric constant

Jpn. J. Appl. Phys., Vol. 45, No. 37 (2006)

K. V. LE et al.

anisotropy is small and negative in the N phase and showsa large change due to N to B2 phase transition. Theseexperimental data are useful for investigating other physicalproperties in terms of parameters such as elastic moduli andanchoring energies.

8ε⊥
6B2NI

ε| |

4105

Acknowledgment
110115This work is partly supported by a Grant-in-Aid for
Scientiﬁc Research (S) (16105003) from the Ministry ofEducation, Culture, Sports, Science and Technology, Japan.

120

Temperature (°C)

Fig. 6. Variation of real part of dielectric constant at 10 kHz withtemperature.

packing in smectic layers. The decrease in "k is caused by
the tilt of molecules with respect to the smectic layer normal,
keeping the molecular transverse dipoles parallel to thesmectic layer (perpendicular to the applied electric ﬁeld).
In summary, the pretilt angle and the optical and dielectricanisotropies of a banana liquid crystal that exhibits the Nphase were measured. It is found that the banana liquid
crystal molecules in the N phase tend to orient parallel tothe spin-coated polyimide surface, even if the polyimide
producing a high pretilt angle for calamitic molecules isused. The value of the order parameter estimated from thebirefringence varies slowly with temperature. The dielectric

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