Incorporation in Langmuir and Langmuir-Blodgett films of symmetric fluorine substituted bent-core liquid crystals: Morphological and optical properties

Molecular mono- and multilayer films of a symmetric fluorine substituted bent-core liquid crystalline (LC) compound with hydrocarbon end-chains were prepared by the Langmuir and Langmuir-Blodgett (LB) technique. Langmuir films were characterized by surface pressure isotherms and Brewster angle microscopy. Likewise LB-films deposited onto amorphous glass substrates were characterized by ultraviolet-visible spectroscopy, the optical second harmonic generation technique and atomic force microscopy (AFM). Results show optimal non-centrosymmetric Z-type LB-film deposition for moderate number of layers (n < 20); thereafter, an apparent collapse of the Z-type structure occurs, giving rise to a decrease of the nonlinear optical (NLO) properties. AFM, NLO and linear optical measurements suggest a uniform molecular coverage on the glass substrates and anisotropic orientational distribution; where, according to NLO-experimental data, the LC-bent-shaped molecules are preferably aligned toward the dipping direction with the net molecular polarization perpendicularly aligned to the substrate layer. This observation leads us to implement a simplified model based on the monomeric rod-like approximation, in order to estimate the significant NLO-tensor components