Molecular aggregation in Langmuir–Blodgett films of azo dye/liquid crystal mixtures

Studies of azo dye/liquid crystal mixtures in monolayers formed at an air–water interface (Langmuir films) and on a solid surface (Langmuir–Blodgett films) were performed. Six azo dyes with various molecular structures and two liquid crystal materials: 4-octyl-4′-cyanobiphenyl (8CB) and trans-4-octyl(4′-cyanophenyl)-cyclohexane (8PCH) were used. The dyes were added to the liquid crystal matrices at various concentrations and the surface pressure–mean molecular area isotherms for the resultant Langmuir films were recorded. On the basis of the isotherm runs, conclusions concerning the molecular organization and the miscibility of the components in the ultrathin films were drawn. Absorption spectra for the Langmuir–Blodgett films, using both natural and linearly polarized light were recorded. Information regarding intermolecular interactions in the mixtures of the nonamphiphilic dye and the liquid crystal, with the polar terminal group was obtained. The ability of dye molecules to form self-aggregates at the air–solid substrate interface was observed. The influence of the molecular structure and the concentration of a dye on aggregates' geometry is discussed. It was found that at low dye concentrations in the liquid crystal J-type aggregates are formed. High dye contents are conducive to the creation of H-type aggregation, which improve the molecular packing in the Langmuir–Blodgett film.