The Aggregation of a Merocyanine Dye in Solution and Film

Two low-energy bands, one near 595 nm and the other near 630 nm, were observed in the visible absorption spectra of a merocyanine dye (3-carboxymethyl-5-[2-(3-octadecyl-2-benzothiazolinylidene)ethylidene]-2-thioxo-4-thiazolidinone) dissolved in methanol–water mixtures containing KOH and Triton X-100. From the dye-concentration dependence on the spectra, the band near 595 nm was assigned to the tetramer, and the one near 630 nm, (probably) to the hexamer of the carboxylate form of the dye. The same bands were observed in the dye films when they were soaked in aqueous solutions of pH≥5.5. The relative intensities of the two bands were strongly affected by the ionic strength of the aqueous solution.