  The development of optically active area consisting of organic molecules are
essential for the devices like optical switches and waveguides, as it can be
easily maneuvered by the application of suitable electromagnetic (EM) waves. In
this article, we report the development of a photoactive surface by the
deposition of a single layer of Langmuir-Blodgett (LB) film of a novel H-shaped
liquid crystal (HLC) molecule. The synthesized HLC molecules possess azo-groups
and nitro-groups. The azo-group can be isomerized (trans-cis transformation) by
irradiating them with ultraviolet (UV) light. The nitro-group can provide
sufficient amphiphilicity to the HLC molecules to form a stable Langmuir
monolayer at air-water interface. The Langmuir monolayer of the HLC molecules
exhibited gas and liquid-like phases. A single layer of LB film of HLC
molecules was deposited on a gold chip of a home-built surface plasmon
resonance (SPR) instrument. The azo-groups of the molecules in LB film was
excited by UV irradiation leading to a change in morphology due to trans-cis
transformation. Such a change in morphology can lead to a miniscule change in
refractive index (RI) of the LB film. SPR is a label free and highly sensitive
optical phenomenon for the measurement of such changes in RI. In our studies,
we found systematic changes in the resonance angle of the LB film of HLC
molecules as a function of intensity of the UV irradiation. We measured
switch-on and switch-off intensity which may suggest that the LB film of HLC
molecules can find applications in optical switches or waveguides.
