Preparation and structural properties of an azobenzene liquid crystal Langmuir-Blodgett film

Azobenzene-containing liquid crystal, 2-hydroxy-4-hexyloxy-4′-hexyloxy azobenzene (AB), was synthesized and used to produce LB films. The surface pressure vs surface area isotherm showed that the liquid crystal molecules formed a stable multilayer on the water subphase owing to the self-assembly for their much smaller area per molecule with 0.1 nm² at π = 0 and 0.07 nm² at collapse point. This spontaneous behavior was also confirmed by small angle X-ray diffraction for a 'horizontally once-transferred' LB film. The 'vertically multi-transferred' LB film also exhibited the same ordered layer structure with the long spacing of the multilayer about 2.0 nm. Polarized FTIR spectra were used to measure the orientation of liquid crystal molecules in LB films with a tilt angle of 60° and 48° (corresponding to azobenzene moiety and hydrocarbon chain, respectively), which is well in accordance with the result of X-ray diffraction. The result from UV-vis absorption spectra revealed that the AB molecule formed H-aggregation in LB films.