Supplementary Material
Photo-driven giant reduction of the Frank elastic constants in a bent-core
nematic liquid crystal
P. Lakshmi Madhuri, S. Krishna Prasad, Uma S. Hiremath and C.V. Yelamaggad
Centre for Nano and Soft Matter Sciences, Jalahalli, Bangalore 560013, INDIA
Materials and Methods
The host BCN compound 4-cyanoresorcinol bis[4-(4-n-hexyloxybenzoyloxy)benzoate
(6OCN for short), exhibits the nematic phase with a TNI (nematic-isotropic transition
temperature) of 135oC, and supercooling down to ~ 90oC [15]. The guest UV-active dopant,
Ethoxyphenylazophenylhexanoate (EPH for short) from Eastman Kodak, is also a nematogen
(although of the calamitic type), facilitating a homogeneous mixture with the bent-core
compound. The molecular structures of 6OCN and EPH are given below.
6OCN
Iso 135oC N 67oC Cr
EPH
Iso 129.3oC N 74.4oC Cr
All the results reported in this paper are on the mixture consisting of 7 weight % of
EPH in the BCN compound. For the determination of the splay (K11) and bend (K33) elastic
moduli, we probed the material permittivity through the electric field driven Freedericksz
transition. Owing to the positive dielectric anisotropy  ε|| – ε⊥, where || and ⊥ indicate the
probing directions parallel and perpendicular to the nematic director n, respectively) of the
sample, a sandwich cell with the molecules oriented in the planar fashion was employed. The
commercial cell of 9 m nominal thickness (Awat, Poland) used for the purpose had the glass
substrate surfaces pre-treated with a polymer layer and unidirectionally rubbed. In this
geometry, an applied electric field applied normal to the substrate plane results in a positive
torque on n causing reorientation of the molecules towards the homeotropic orientation well
above a certain threshold voltage, Vth. This results in the permittivity changing from ε⊥ for
V<Vth to ε|| for V>>Vth. The apparatus for the permittivity measurements includes a precision
LCR meter (Agilent E4284A) operated at a frequency of 1 kHz, and capable of applying an
ac voltage up to 20 V. Photoisomerization effects were brought about by employing an
intensity-stabilized UV source (Hamamatsu LS5) with a fibre-optic light guide in conjunction
with the combination of an IR filter and wavelength-determining (365 nm) UV filter. The
actual intensity received by the sample was measured using a UV-power meter (Hamamatsu,
C6080-03).
0.60

mix-with uv
0.45
mix-no uv
0.30
mix-no uv
n
0.12
mix-with uv
0.09
0.06
0
-5
-10
-15
-20
o
T-TC ( C)
Figure S1: Temperature dependence of the (a) dielectric anisotropy  and (b) birefringence
(n) in the nematic phase for the mixture, without and in the presence of UV illumination of
IUV = 5 mW/cm2. The two parameters behave in opposite ways, with  increasing, and n
decreasing upon photoisomerization, although the change in the former parameter is much
larger.