Molecular arrangement in Langmuir and Langmuir-Blodgett films of a mesogenic bent-core carboxylic acid.

ABSTRACT

A different alternative to previous research on Langmuir and Langmuir-Blodgett (LB) films of bent-core liquid crystals is reported in this work. A bent-shaped molecule wearing a terminal carboxylic group has been used to obtain monomolecular films with their long molecular axis almost perpendicular to the aqueous surface. Langmuir films at the air-liquid interface (pH=9) have been characterized by a combination of surface pressure and surface potential versus area per molecule isotherms, Brewster angle microscopy, and ultraviolet reflection spectroscopy. A condensed phase is reached at surface pressures up to 20 mN x m-1. In this condensed phase, molecules are packed forming H-aggregates with a well-defined molecular orientation. Langmuir films have been transferred onto quartz and silicon substrates and characterized by means of UV-vis spectroscopy and XRR. The transference is Z-type, with a constant deposition of the monolayers. The total LB monolayer film thickness is evaluated to be about 5.8 nm, which is in good agreement with the deduced orientation at the air-liquid interface as well as with the lamellar order observed within the solid obtained by cooling the sample from the mesophase.