New optical properties by extraordinary high oscillator strength in ordered dye films

Molecular interactions in crystals and highly ordered films of the cyanine dye 1.7-bis(dimethylamino)heptamethine were investigated by polarized UV/VI S spectroscopy. The films of 80 nm thickness and a uniform crystalline orie ntation in large areas were prepared by a new spherulitic crystallization p rocess. This process allowed a variation of both the arrangement of chromop hores in the crystal lattice as well as the inclination of the crystal latt ice in respect to the substrate. Seven polymorphs could be prepared, which exhibit quite different dichroic colors. The largest energy splitting of 12 100 cm(-1) between the dichroic absorption maxima was observed for the gamm a -polymorph. Spherulites of this polymorph exhibit three differently color ed regions. Their unusual dichroic absorption spectra could be explained by means of investigations of the structure and the optical properties of sin gle crystals of the same dye. The absorption is determined by strong excito nic interactions and by the directional dispersion of polaritons. The excit onic character of the absorption was proved by photobleaching of the crysta l transitions by light of selected wavelength and polarization. The especia lly strong cooperative interactions in these materials lead to quite new pr operties which has a high potential for applications in advanced optical de vices.