Electronic Spectra of Single Crystals of 1,1′-Diethyl-2,2′-Cyanine Iodide, Bromide, and Chloride

Reflection spectra of single crystals of 1,1′-diethyl-2,2′-cyanine iodide, bromide and chloride were measured in order to find the origin of the J band. The spectra were obtained by the Kramers-Kronig transformation of reflectivity. The positions of absorption peaks and the shapes of vibronic envelope for each crystalline faces are explained by a calculation of surface exciton splitting with a weak coupling model. The intensity of each bands is interpreted by the oriented gas model, however, a peculiar anomalous band shape characteristic of J band is explained by the charge resonance (CR) and the molecular exciton interaction. The J band is found along particular crystalline direction, in which the dye molecules are stacked face to face, showing importance of interplanar π electron interaction. The molecular exciton band shape is changed significantly to give a J band because the higher vibronic bands are blurred off by the enhanced relaxation mechanism of the CR interaction.