Opto-dielectric effect on a nematic liquid crystal doped with a photoactive
azo mesogen
Geetha G. Nair, S. Krishna Prasad,
a)
and C. V. Yelamaggad
Centre for Liquid Crystal Research, Jalahalli, Bangalore 560 013, India
͑Received 9 August 1999; accepted for publication 16 November 1999͒
We report the detailed temperature variation of the dielectric constant across the nematic-isotropic
transition in both the normal and UV-irradiated states of a commercially available room temperature
nematic mixture ͑E7 from E-Merck͒ doped with a small quantity ͑4.5 mol %͒ of a liquid crystalline
azo compound. The photoirradiation was done using a very low power ͑0.1 mW/cm
2
͒ UV source.
Data of time-resolved measurements of the dielectric constant through both the UV-activated
‘‘trans-cis’’ transformation and the thermal back relaxation process are also presented and
compared with time-dependent absorbance data. Even for this low concentration of the photoactive
material, UV irradiation of as small a magnitude as 0.1 mW/cm
2
is observed to lower the
nematic-isotropic transition temperature by 15°C. © 2000 American Institute of Physics.
͓S0021-8979͑00͒00405-9͔
I. INTRODUCTION
Photonics, in which light can be controlled by light as a
stimulus, is being proposed as the future technology for
high-speed information processing.
1
It is well known that
liquid crystals are convenient to control light because they
exhibit large optical effects owing to reorientation of the
molecules with the application of low-magnitude electric
fields.
2
It has been shown
3
that even in nonphotoabsorbing
liquid crystals, optical fields, albeit large, can be used to
cause a similar reorientation. However, an addition of chro-
mophores, such as azo groups, has been found to lower the
required optical field significantly.
4
Furthermore, the photo-
induced birefringence has extremely high long-term stability
and holograms recorded show hardly any degradation. This
feature has made the photochromic materials, in particular
the azo compounds, very promising candidates for optical
data storage. Several groups have done extensive work in
this field.
5–12
Molecules containing azo groups are well known to
show reversible isomerization transformations upon irradia-
tion with UV and visible light. Upon absorption of UV light
͑typically 365 nm͒, the energetically more stable trans con-
figuration with an elongated rod-like molecular form, trans-
forms into a bent banana-like cis configuration. The reverse
transformation of the cis isomer into the trans isomer can be
brought about by irradiation of visible light ͑in the range of
400–500 nm͒. The latter change also occurs in the ‘‘dark’’
by a process known as ‘‘thermal back relaxation’’ in a period
ranging from minutes to tens of hours depending on the sys-
tem.
Photoinduced effects reported in the literature are on liq-
uid crystals in which the azo group is either chemically at-
tached to the molecule or is used as a dopant in a liquid
crystal host material. In the case of the latter, guest-host sys-
tems, the trans form of the azo dopant, as it is rod-like, is
favorable for the stabilization of the liquid crystalline phase.
On the other hand, the cis form, which is in bent form acts
like an ‘‘impurity’’ and therefore destabilizes the liquid crys-
talline phase. Hence, photoisomerization from the trans to
the cis form causes, in general, a lowering of the transition
temperature. If, for instance, the material exhibits a nematic-
isotropic (N–I) transition and the UV irradiation is done in
the nematic phase, the lowering of the transition temperature,
T
NI
, could induce an isothermal N–I transition. It is this
photochemically induced transition that is promising for the
optical image-storing systems.
5
One of the requirements for
such an application is that the lowering of T
NI
be
appreciable.
5,7,12
Although, large shifts in T
NI
can be ob-
tained if the host material itself is photoactive,
13
from the
operational point of view, it is obviously better to dope small
quantities of guest photoactive material into a host liquid
crystalline system which has been optimized for temperature
range, viscosity, etc. In this article, we report the detailed
temperature variation of the dielectric constant across the
N–I transition in both the normal and UV-irradiated states
on a commercially available eutectic mixture ͑E7, from
E-Merck͒ exhibiting a nematic phase, which is doped ͑4.5
mol %͒ with an azo compound, which is also liquid crystal-
line. We also present data of time-resolved measurements of
the dielectric constant through both the UV-activated trans-
cis transformation and the thermal back relaxation process.
The variation in the latter process is compared with the re-
verse ͑cis-trans͒ isomerization rate. It should be emphasized
that all these measurements have been done using a very low
UV power of 0.1 mW/cm
2
from a conventional source. Even
with this low power and low concentration of the photoactive
material, we have observed a 15 °C shift of T
NI
, a value
comparable to those obtained by much higher power of the
radiation.
5,10
a͒
Electronic mail: uclcr@giasbg01.vsnl.net.in
JOURNAL OF APPLIED PHYSICS VOLUME 87, NUMBER 5 1 MARCH 2000
20840021-8979/2000/87(5)/2084/6/$17.00 © 2000 American Institute of Physics