Huge splitting of dichroic absorption energies in ordered cyanine dye films

Abstract

Molecular interactions in highly ordered films of the cyanine dye 1,7-bis(dimethylamino)heptamethine tetrafluoroborate were investigated by polarized UV/VIS spectroscopy. The films (80 nm) were prepared by a spherulitic crystallization process, in which both the arrangement of chromophores in the crystal lattice and the inclination of the crystal lattice in respect to the substrate could be varied. Seven different polymorphs were observed. The spectroscopic properties of the γ-form, having the largest splitting of the dichroic absorption energies of 12100 cm^-1, were studied. This form exhibits three different crystal faces, showing quite different colors although the crystal structure is the same. The unusual absorption properties could be explained by way of superstructure–absorption relationships, derived from single crystals of this dye. The layer absorption is determined by strong excitonic interactions and by the directional dispersion of polaritons. The excitonic character of the absorption was proved by photobleaching of the crystal transitions by light of selected wavelength and polarization.