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 (epsilon(M) and epsilon(D)), the dimer association constant (K-D) and the dimerization enthalpy (DeltaH(D)) were determined studying dye concentration and temperature dependencies of absorption spectra. In 0.01 mol/dm(3) 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 (epsilon(J)) and the J-aggregatc association constant (K-J) were quantitatively estimated from a modified Harris-Hobbs equation. The J-aggregates were found to be oligomers composed of 20-80 monomers. K-D and DeltaH(D) suggested that substituents at the 5,5'-position influenced dimer formation and played an important role in aggregates formation. (C) 2004 Elsevier Ltd. All rights reserved.