Spectral properties of aggregates in adsorbed dye monolayer films

Abstract

Adsorbed dye monolayer films are prepared by the complexation of a cyanine dye dissolved in the aqueous subphase to stearic acid (SA), oleic acid (OA) and dihexadecyl phosphate (DHP) monolayers at the air-water interface. Using UV-Vis spectroscopy, a wide range of aggregation behaviour of the dye was observed with variation of the subphase pH and pre-treatment of the quartz substrate onto which the monolayers were transferred from the water surface. The dye complexed to a 1:1 molar mixture of SA and OA showed both monomer and J-aggregate on a hydrophobic substrate and only monomer when transferred onto a hydrophilic substrate. For the DHP/dye monolayers, changing the subphase pH and the surface pressure at transfer yielded differing amounts of monomer and aggregate, with the greatest amount of aggregate being obtained when transferring from the condensed phase. Furthermore, two different types of J-aggregate for the DHP/dye systems were observed depending upon the substrate pre-treatment.