STRUCTURAL CONTROL OF OPTICAL-SPECTRA OF 2-DIMENSIONAL MIXED DYE CRYSTALS

Optical properties (absorption and fluorescence spectra) with their connection to the structure of mixed cyanine dye crystals were investigated. The latter were prepared by adsorption of water-soluble dyes to the oppositely charged surface of a lipid monolayer during compression at the air-water interface. Emission spectra and the morphology of such systems were observed in situ during the formation of crystals at the water surface. Electron diffraction data are compared with that obtained from calculated intensities of a two-dimensional molecular arrangement. Hence, not only the lattice constants but also the relative orientation of the two molecules per unit cell can be determined. The angle between the two differently oriented molecules is obtained independently from the optical absorption spectra by assuming two excitonic transitions (Davidov splitting). It was shown that the structural parameters can be varied by preparing mixed crystals with appropriate composition of slightly different dyes, and thus also the optical properties can be changed while maintaining sharp absorption edges.