Langmuir–Blodgett films of dichroic dyes oriented by liquid crystal

Abstract

A cyanobiphenyl molecular film mixed with nonamphiphilic azobenzene and benzimidazole derivative dichroic dyes was transferred from an air–water interface onto a hydrophilic quartz plate. 4-cyano-4-octylbiphenyl is a liquid crystal material which played the role of a host in the mixed films. The mole fraction of dye to cyanobiphenyl molecules was varied from 0.5 to 0.02. One layer was successfully deposited onto a quartz plate with the transfer ratio slightly higher than one. The films were deposited at a surface pressure π between 1 and 15 mN/m. The structure of the film at the air–water interface was observed by using a Brewster angle microscope. The orientation of cyanobiphenyl as well as dye molecules were estimated by polarized absorption spectra. The angle of the cyanobiphenyl (β H ) molecules between the long molecular axis and normal to the quartz surface in pure and mixed monolayers was found to be between 64° and 54°. These values depend on the surface pressure during deposition. The smaller angles were observed for mixed layers and for high surface pressure. Analyzing polarized spectra of dyes incorporated in the monolayer made similar observations. The dyes show a sharper angle β G =(60°–38°) than host molecules (β H =(64°–54°) . The dye absorption spectra at the high dye concentration and surface pressure differ from those for the films deposited at low concentration or low surface pressure.