Herringbone structure in two-dimensional single crystals of cyanine dyes. II. Optical properties

Abstract

Optical spectra of two-dimensional single crystals of cyanine dye molecules are studied experimentally and theoretically. Crystallites of three carbocyanine dye molecules, which are chemically different, but exhibit nearly identical optical properties, are prepared by adsorption from aqueous solution at a charged ammonium lipid monolayer. By means of electron diffraction it was found that the crystal structure of the aggregates consists of a rectangular unit cell containing two molecules of different orientation. Measurements of polarized absorption, reflectance, and fluorescence spectra of single crystallites reveal that this structure leads to Davydov splitting of the electronic transition into two exciton bands. The band structure of these exciton bands is calculated with the help of Kuhn’s extended dipole model. It is demonstrated, how the exciton energy depends on the relative orientation of the two molecules within the unit cell. Calculated splitting energies are in reasonable good agreement with experimentally observed values.