Substituent effects on aggregation phenomena in aqueous solution of thiacarbocyanine dyes

Abstract

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 ( ɛ M and ɛ D), the dimer association constant ( K D) and the dimerization enthalpy (Δ H 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 K D value. In addition, the aggregation number, the molar extinction coefficient of the J-aggregate ( ɛ J) and the J-aggregate 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 Δ H D suggested that substituents at the 5,5′-position influenced dimer formation and played an important role in aggregates formation.