Optical properties of a streptocyanine dye single crystal with tubelike aggregation: bis(dimethylamino)-heptamethinium tetrafluoroborate BDH+BF−4

Abstract

Normal incidence reflectance spectra from different faces of an optically biaxial dye single crystal, bis(dimethylamino)-heptamethinium tetrafluoroborate, have been measured for two orthogonal polarizations of light. The spectra are very anisotropic with a wide band of quasi-metallic reflectance for one polarization of light. Strong coupling of light to electronic states leads to splitting of transverse and longitudinal excitons as large as 1.5 eV, resulting in strong directional dispersion of the spectra. The data were analyzed by Kramers-Kronig transformation of spectra from appropriate faces and checked by comparison of solutions of the wave equation and the experimental spectra. Electronic transition energies of two Davydov components were found at 2.07 and 2.18 eV shifted by 0.32 and 0.24 eV with respect to absorption spectra of molecules in solution. The results are compared with those obtained from the chloride of the same dye molecule.