Substituent effects on aggregation phenomena in aqueous solution of thiacarbocyanine dyes

Visible absorption and fluorescence spectra of seven thiacarbocyanine dyes with varying substituents at 5,5′- and meso-positions have been studied suggesting J-aggregate formation in aqueous solutions. The molar extinction coefficients of monomers and dimers (ɛM and ɛD), the dimer association constant (KD) and the dimerization enthalpy (ΔHD) were determined studying dye concentration and temperature dependencies of absorption spectra. In 0.01 mol/dm3 KCl aqueous solutions, J-aggregates were observed due to the association of dimers at a critical dye concentration, which correlated with the KD value. In addition, the aggregation number, the molar extinction coefficient of the J-aggregate (ɛJ) and the J-aggregate association constant (KJ) were quantitatively estimated from a modified Harris-Hobbs equation. The J-aggregates were found to be oligomers composed of 20-80 monomers. KD and ΔHD suggested that substituents at the 5,5′-position influenced dimer formation and played an important role in aggregates formation.