In situ detection of birefringent mesoscopic H and J aggregates of thiacarbocyanine dye in solution

Polarized-light microscopy, fluorescence microscopy, atomic force microscopy as well as absorption and fluorescence spectroscopy were used to characterize mesoscopic structures of both supramolecular H and J aggregates of 3,3'-disulfopropyl-5,5'-dichloro-9-methyl thiacarbocyanine dye in aqueous solution. Polarized-light microscopy visualizes in situ the mesoscopic morphology of the H and J aggregates and distinguishes between them by their own colors. The H aggregate having a fibrous structure showed negative birefringence, namely, the refractive index along the fiber short axis was higher than that of the long axis, so that pi-electron chromophores of the dye molecule are likely to orient along the short axis of the elongated fibers. The degree of birefringence of the H aggregate fiber was similar to-0.3. Investigations on the concentration dependence of the absorption spectra showed that the amount of J aggregates increased at the expense of a decrease in the amount of H aggregates. With respect to the J aggregates, a small dot morphology was observed at a relatively low dye concentration of 3.0 mM. With an increase of the dye concentration up to 10 mM, the morphology changed into mesoscopic fibers. In contrast, fluorescence microscopy for the fibrous J aggregates reveals that the constituent molecules are approximately aligned along the long axis of the fibers.