Analysis of counter ions of cyanine dye in Langmuir-Blodgett films by PIXE

Langmuir-Blodgett (LB) films of 1-methyl-1′-octadecylquinocyanine bromide, iodide and perchlorate were prepared from subphases containing NaClO₄, KBr and/or KI. Each sample consists of an arachidic cadmium layer and about 100 cyanine dye layers on a Si substrate. Anions incorporated into the film as counter ions from the subphase were analyzed by particle induced X-ray emission (PIXE) using a 2.5 MeV He²⁺ beam. It was found that the dissociation of the cationic dye molecule in the monolayer at the air/water interface leads to the substitution of counter anions of the dye molecules, and the subsequent incorporation of the counter anions in the LB films. Some of the dissociation constants of cyanine halide dyes were estimated.