  In the present communication we report the preparation and characterizations
of Langmuir and Langmuir-Blodgett films of a thiacyanine dye-dioctadecyl
thiacyanine perchlorate (NK) mixed with Octadecyl trimethyl ammonium bromide
(OTAB). The relationship between the molar ratio of OTAB and NK and the
orientation of molecules at the air-water interface was investigated using
surface pressure - area per molecule ({\pi}-A) isotherm. UV-Vis absorption and
fluorescence spectroscopic investigations reveal that prominent J-aggregation
of NK molecule was observed in the LB films lifted at higher surface pressure.
This J-aggregation can be controlled by diluting the NK molecules with OTAB. It
was observed that the J-aggregates of NK decayed to monomer and H-aggregates
when the NK-LB film was exposed to a monochromatic light of wavelength 460 nm
(Lambda max of J - aggregates).
