Materials Sciences and Applicatio n, 2011, 2, 839-847
doi:10.4236/msa.2011.27114 Published Online July 2011 (http://www.SciRP.org/journal/msa)
Dielectric Behaviour of Pure and Dye Doped
Nematic Liquid Crystal BKS/B07
Rajiv Manohar2, Shashwati Manohar3, V. S. Chandel1
1Department of Physics, Integral University, Lucknow, India; 2Department of Physics, Lucknow University, Lucknow, India;
3Department of Physics, RML Awadh University, Faizabad, India.
Received July 26th, 2010; revised September 4th, 2010; accepted May 30th, 2011.
ABSTRACT
The dielectric properties of pure nematic liquid crystal (BKS/B07) and dye doped (Rhodamine B and Anthraquinone)
nematic liquid crystal have been investigated in a wide frequency range of 1 kHz to 10 MHz through the dielectric
spectroscopic method at varying temperature. In addition to this optical transmittance and textures of the samples have
also been observed with a polarizing microscope.
Keywords: Liquid Crystals, Dielectric Properties
1. Introduction
Organic molecules and liquid crystals (LC) are well es-
tablished materials for photonic and non-linear optical
device applications because of their optical non linearity
and very rapid optical response [1]. In a mixture of LC
and dichroic dye the action of an electric field influences
the collective orientation of the LC molecules and that of
the dye molecules. This phenomenon is called Guest-host
interaction. Dyes have been widely used as guest addi-
tives in optical materials to develop novel optoelectronic
devices. Again it is well established that the presence of
dye molecules in the liquid crystal host influences many
properties of the pure liquid crystals. The Effect of dye
on dielectric properties of liquid crystals has been inves-
tigated earlier by some groups [2,3]. The change in
nonlinear refractive index and birefringence of dye doped
nematic mixture has been investigated by A. Jafari et
al.[4]. It has also experimentally proved that doping a
small amount of dye decreases the required threshold of
molecular reorientation further in LC materials [5].
Temperature dependence of direction reorientation of
dye doped nematic liquid crystal has been studied by
Esteves et al. [6]. Voigot et al. have studied emission of
circularly polarized light in dye based chiral nematic
liquid crystals [7]. Thermal non-linearity of dye doped
nematic liquid crystals have been reported by Henninot
et al. [8], while photovoltaic effect in liquid crystal cells
containing dyes have been studied by Sato et al. [9].
Mustafa Okutan et al. have studies refractive index dis-
persion and electrical properties of carbon nono –balls’
doped nematic liquid crystal [10].
Most of the efforts are aimed on the electro-optical
characterization of LC [11,12], but no serious work have
been done in understanding the dielectric properties of
dye doped liquid crystals. In the recent years few articles
have been published by Mustafa Okutan et. al., S. Ozder
et al. [13-15].The dielectric spectroscopy technique has
also been used by many workers for the study of LCs in
different phases, as the technique provides very accurate
data about the material under investigation. Therefore,
the present paper is an effort to investigate the effect of
dye on dielectric property of liquid crystal host mole-
cules. In order to confirm the changes occurred in phase
transition temperature as a result of dye mixing optical
transmittance measurement of the sample have also been
done.
2. Materials
The liquid crystal sample used for the present investiga-
tion was used without further purification. The transition
schemes of the pure sample as well as dye doped samples
are as follows: (Figure 1)
All dyes used in the experiment were obtained from
Thomas Beker. Their chemical names and compositions
are given below. (Figure 2)
3. Experimental
The pure sample was used as such and the dye doped

Dielectric Behaviour of Pure and Dye Doped Nematic Liquid Crystal BKS/B07 840
Crystalline 126 C


 Nematic 130 C


 Isotropic
Nematic sample doped with anthraquinone dye
Crystalline 121 C


 Nematic 125 C


 Isotropic
Nematic sample doped with rhodamine B dye
Crystalline 122 C


 Nematic 126 C


 Isotropic
Figure 1. Structure of Nematic sample BKS/B07.
Anthraquinone Dye
1,5- Diamino-2(4-heptyloxy-phenyl)-4,8-dihydroxy-anthraquinone
CHEMICAL COMPOSITION: C27 H23 N2 O5
OOH
NH
2
NH
2
OH
OC
7
H
15
O
(a)
Rhodamine B Dye
N-[9-(2-Carboxyphenyl)-6-(diethylamino)-3H-xanthen-3-ylidene]-
N-ethylethanaminium chloride
CHEMICAL COMPOSITION: C28 H31 ClN2 O3
N
+
O
N
OOH
C
l
-
(b)
Figure 2. Structures of dyes used in present study.

Dielectric Behaviour of Pure and Dye Doped Nematic Liquid Crystal BKS/B07 841
6810 12 14 16 18
3
4
56810 12 14 16 18
2
3
4
5
6
7
86810 12 14 16 18
4
5
6
7
8
Pure BKS/B07 12C60
1280C
log10 of Frequency
'BKS/B07 doped with anthraquinone dye 124C
0
1250C
BKS/B07 doped with rhodamine B dye 1240C
1250C
124˚C
125˚C
124˚C
125˚C
126˚C
8˚C 12
Figure 3. Variation of dielectric constant with log10 of frequency for pure nematic sample with two dyes (anthraquinone,
rhodamine B)
6810 12 14 16 18
0
1
26810 12 14 16 18
0
1
2
36810 12 14 16 18
0
1
2
3
Pure BKS/B07 1280C
1260C
log10 of Frequency
"BKS/B07 doped with anthraquinone dye 1240C
1250C
BKS/B07 doped with rhodamine B dye 1240C
1250C
124 C˚
˚C 125
124˚
125˚
C
C
128˚C
126˚C
Figure 4. Variation of dielectric loss with log10 of frequency for pure nematic sample with two dyes (anthraquinone, rhoda-
mine B).

Dielectric Behaviour of Pure and Dye Doped Nematic Liquid Crystal BKS/B07 842
sample of BKS/B07 was prepared in the lab by disper-
sion of two dyes under investigation at a concentration of
about 2% wt/wt in the liquid crystal host material. The
strong interaction between two different types of mole-
cules allow the samples to maintain a good alignment
[16], which has been confirmed by observing at the sam-
ple holder under polarizing microscope. This guest-host
mixture and pure sample were filled in a sandwich type
sample holder having coating of Indium Tin Oxide (ITO)
on the glass substrate and pre-treated with polyamide for
planner type alignment. The temperature of the sample
was controlled and maintained by a microprocessor
based temperature controller Julabo (model no. F-25 HD)
to an accuracy of  0.01˚C. The temperature of the sam-
ple holder on the double walled jacket was measured
using an external Pt-100 type sensor. The sample cell
was connected to Hewlett-Packard Impedance/Gain
Phase Analyser (model HP 4194 A) working in the fre-
quency range of 1 kHz to 40MHz. For obtaining the val-
ues of capacitance and dissipation factor, the sample
holder was calibrated using AR grade benzene. The val-
ues of capacitance in the cell with sample and without
sample were measured. The dielectric constant was ob-
tained using the relation
'
G
C
C


1
(1)
where ∆C is change in capacitance of the sample holder
with and without sample, which is
0P
CC C (2)
where
P
C is the capacitance of sample holder with
sample and 0 C is the capacitance of sample holder
without sample. is the geometrical capacitance of
the sample holder.
G
C
For the measurement of dielectric loss, dissipation
factor D was measured for the sample holder with and
without sample. Thus, the loss tangent was obtained us-
ing the relation
tan
P
POO
PO
CD CD
CC



 (3)
where
P
D and O are the dissipation with and with-
out sample respectively.
D
Dielectric loss was determined using relation
ε= εtanδ (4)
For the measurement of optical transmittance, a preci-
sion research polarising microscope was used (model no.
CENSICO 7626). The homogeneously aligned sample
holder was placed on a hot stage which is a double
walled chamber in which oil of required temperature was
circulated. For variation of temperature, the same tem-
perature controller Julabo (model no. F-25 HD) as used
during dielectric measurements was again used. The hot
stage was placed between cross polarisers in the polaris-
ing microscope. The aligned sample was placed between
crossed polarisers and was heated and the emergent light
at different temperature was made to incident on one of
the eyepieces which is fitted with a light dependent re-
sistance (LDR). Thus, the variation of emergent light
varies the resistance of LDR, which directly provides us
the optical transmittance in arbitrary units.
Textures (microphotographs) of the samples have also
been taken using a polarising microscope which is fitted
with a camera in one of its eye pieces.
4. Results and Discussion
The variation of real and imaginary parts of relative per-
mittivity, that is dielectric constant (  ) and dielectric loss
(
 ) for pure nematic sample BKS/B07 and two dye
mixed samples with log of frequency are shown in Fig-
ures 3 and 4 respectively. The variation of real and
imaginary parts of relative permittivity that is dielectric
constant (  ) and dielectric loss (  ) for pure nematic
sample BKS/B07 and with two dyes anthraquinone
(mixture 1) and rhodamine B (mixture 2) with tempera-
ture are shown in figures 5 and 6 respectively. Figure 7
shows the variation of percentage optical transmittance
with temperature for pure nematic sample BKS/B07 and
two dye mixed samples (mixture 1 and 2). Textures of
the nematic sample BKS/B07 along with its two guest-host
mixtures in crystalline and nematic phases have been
presented in Figures 8 to 9.
The dielectric parameters   and   have been meas-
ured for a nematic liquid crystal BKS/B07 in the fre-
quency range of 1 kHz to 10MHz for the temperature
range of 70˚C to 135˚C. Figure 3 and Figure 4 represent
typical frequency dependence spectra of the real and
imaginary part of the dielectric permittivity measured for
nematic sample BKS/B07 and dye doped mixtures 1 and
2. The dielectric permittivity is found to be either con-
stant or to decrease as the frequency increases [17-22] for
pure sample. Lower values of   at higher frequency
suggest that the molecules rotate about their long mo-
lecular axis [23]. The behaviour of   for the dye based
mixtures 1 and 2 is similar to that of pure sample but the
values of dielectric constants are higher in comparison to
the corresponding values of pure sample.
One can see from the figure 4 that a maximum in the
loss curve is observed at frequency 382 kHz for the tem-
perature 128˚C for pure sample. The peak in the curve
indicates dielectric relaxation and it shifts towards lower
side of frequency as the temperature increases as ex-
pected and also reported by other workers [24,25].
The nature of loss curve again is similar to that for
pure sample for other dye doped samples but it is evident

Dielectric Behaviour of Pure and Dye Doped Nematic Liquid Crystal BKS/B07 843
100 110 120 130
3.2
3.6
100 110 120 130 140
4.0
4.4
4.8
5.2
100 110120 130140
4.0
4.4
4.8
5.2
5.6
6.0
Pure BKS/B07
100 KHz
200 KHz
'
100 KHz
200 KHz
BKS/B07 doped with rhodamine B dye
BKS/B07 doped with anthraquinone dye 100 KHz
200 KHz
Temperature (℃)
Figure 5. Variation of dielectric constant with temperature for pure nematic samplewith two dyes ( anthraquinone, rhoda-
mine B).
100 110 120130 140
0. 5
1. 0
100 110 120130 140
0. 5
100 110 120130 140
0. 5
1. 0
Pure BKS/B07
100 KHz
200 KHz
"
BKS/B07 doped with rhodamine B dye
100 KHz
200 KHz
BKS/B07 doped with anthraquinone dye
100 KHz
200 KHz
Temperature (℃)
Figure 6. Variation of dielectric loss with temperature for pure nematic samplewith two dyes ( anthraquinone, rhodamine B)

Dielectric Behaviour of Pure and Dye Doped Nematic Liquid Crystal BKS/B07 844
100 105 110 115 120 125130 135 140
0
10
20
30
40
50
60
100 105 110 115 120 125130 135 140
0
10
20
30
40
50
60
70
80100 105110 115 120 125130 135 140
0
10
20
30
40
50
60
70
80
Pure BKS/B07
% Inverse Optical Transmittance
BKS/B07 doped with anthraquinone dye
BKS/B07 doped with rhodamine B dye
Temperature (℃)
Figure 7. % Inverse Optical transmittance Vs temperature for pure nematic samplealong with two dyes ( anthraquinone,
rhodamine B).
(a) (b)
Figure 8. Texture of pure nematic sample BKS/B07 in (a) crystalline phase at 80˚C; (b) nematic phase at 127˚C.
(a) (b)
Figure 9. Texture of Pure nematic Sample BKS/B07 doped with anthraquinone dye in (a) crystalline phase at 80˚C; (b)
nematic phase at 123˚C.

Dielectric Behaviour of Pure and Dye Doped Nematic Liquid Crystal BKS/B07 845
(a) (b)
Figure 10. Texture of Pure nematic Sample BKS/B07 doped with rhodamine B dye in (a) crystalline phase at 80˚C;(b)
nematic phase at 123˚C.
from the Figure 4 that the peak of the curve has been
significantly shifted from the frequency of the peak ob-
tained for pure sample. It suggests that the relaxation
frequency of the dye doped mixture is different from that
of pure sample and variation also depends upon the type
of dye used and its compatibility with the liquid crystal
host.
The variation in the value of
 and   due to addition
of dye can be explained by considering the contribution
of dipole moment of dye molecule in the host liquid
crystal molecule. Each dye molecule has a preferential
axis of orientation and it will get aligned along the host
liquid crystal molecule thereby resulting in the restriction
on the freedom of its movement. Thus, the dipole mo-
ment of dye molecule contributes significantly to the
actual value of
  and   and also relaxation frequency
value [26].
The Figures 5 and 6 show the variation of   and   as
a function of temperature for nematic sample BKS/B07
and for mixtures 1 and 2. For the frequency range 1 kHz
–10 MHz, the dielectric constant remains almost constant
for the crystalline region and crystalline to nematic tran-
sition is detected by slight variation of dielectric constant
at 126˚C for nematic sample BKS/B07. It is interesting to
note that a sharp peak appears near nematic to isotropic
transition temperature at 130˚C whose magnitude in-
creases with decreases in the frequency. This type of
variation has also been reported by other workers for
other type of samples [24]. Similar types of variations
have been noticed for the curves of
  versus temperature
at varying frequencies shown in figure 6. The hump at
the nematic to isotropic phase transition temperature is
more clear for lower frequencies.
The variation of
  and   with temperature for
guest-host mixtures also have similar nature with a
change that the transition temperature have shifted to-
wards lower side by about 4˚C to 5˚C in comparison to
the transition temperature for pure samples [26]. The
transition temperature for mixture 1 are 121˚C and 125˚C
for crystalline to nematic and nematic to isotropic transi-
tion. While for the mixture 2 the values of transition
temperatures are 122˚C and 126˚C for the two transitions
respectively. This decrease in the transition temperature
is due to the fact that dye molecules behave as an impu-
rity molecule in the liquid crystal host. The presence of
impurity definitely decreases the transition temperature
of the sample. It has been reported by our group [27]
and various other researchers. Also the increased breadth
of transition suggests that dye molecules behave as im-
purity in the liquid crystal matrix [28].
Figure 7 shows the variation of transmitted light
through the sample in the terms of resistance of LDR
under crossed polariser conditions. The percentage in-
verse optical transmittance decreases and the resistance
increases for the two transitions i.e. crystalline to nematic
and nematic to isotropic. It is obvious that the inverse
optical transmittance remains almost constant in the
crystalline region then it suddenly increases at 126˚C
indicating crystalline to nematic phase transition. It re-
mains constant in nematic region and then again in-
creases significantly by a large amount at 130˚C indicat-
ing nematic to isotropic phase transition after which the
value of transmittance again becomes almost constant
with increase in temperature. This type of behaviour has
been reported by our group [29] and by other workers for
cholesteric as well as nematic liquid crystals [30]. For
dye doped samples, the value of transition temperatures
for nematic-isotropic transitions are 125˚C and 126˚C for
mixture 1 and mixture 2 respectively while the crystal-
line to nematic transition is almost smooth as measured
with the help of optical transmittance measurement tech-
nique. The absorption for two dye doped samples in-
creases by a large amount, which is indicated by the
higher values of the resistance of LDR in the isotropic
region in comparison to the pure sample. However, the
phase transition temperature for two guest-host mixtures
measured using polarising microscope are not sharp as
for the pure (single component) samples. It may be pos-

Dielectric Behaviour of Pure and Dye Doped Nematic Liquid Crystal BKS/B07 846
sible that the dye molecules are behaving like impurity
molecules for liquid crystal host. The breadth of the
nematic-isotropic phase has also been observed by De-
mus et al. [31] and Singh et al. [32] for mixtures. It may
be concluded that sharpness of the transition depends
upon purity of the sample and it increases with the addi-
tion of different components because they behave like
impurity with respect to each and that might be the rea-
son for existence of bi-phase region. The phase transition
temperatures as obtained by dielectric measurement
technique and optical transition measurement technique
are found to be very close to each other.
Textures (microphotographs) of pure sample and their
guest host mixtures are shown in Figures 8(a, b), 9(a, b)
and 10 (a, b) in crystalline and nematic phases. It may be
seen that these textures also support our observations of
the existence of different phases and identification of
phase transitions [33-37].
5. Conclusions
It may be concluded that dielectric constant and dielec-
tric loss values increase with the addition of dye and the
relaxation frequency shift towards the lower or higher
side depending upon nature, structure of dye and com-
patibility with the liquid crystal host. The transition tem-
perature shifts towards lower side with the addition of
dye because the guest molecule behaves like impurity for
the pure sample. The absorption of light increases sig-
nificantly due to presence of guest dye molecule in host
liquid crystal molecule.
ACKNOWLEDGEMENT
Authors are thankful to Prof. B. K. Sadashiva of Ra-
man Research Institute, Bangalore, India for providing
the nematic sample. Authors are also thankful to ISRO
for grant funds in the form of project.
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