Complex behavior of hemicyanine in Langmuir-Blodgett films revealed by surface pressure measurements and fluorescence microscopy

We studied in this work the organization of a hemicyanine dye in Langmuir monolayers and Langmuir-Blodgett (LB) films. Surface pressure measurements revealed the complex behaviour of this molecule at the air-water interface, since three different π-A compression isotherms were obtained under similar experimental conditions. The probability to get one type of isotherm as compared to the others depends on the amount of salts added to adjust the subphase pH, these salts acting as “structure breakers”. Absorption spectra of hemicyanine LB films reveals under specific experimental conditions the presence of H-aggregates into the films, molecules in aggregates being characterized by a particular orientation as compared to surrounding monomers. Their fluorescence images are also different according to corresponding π-A isotherms and pH subphase, showing domains with specific patterns and organization.