SPECTRAL INVESTIGATION OF THE MONOMER-DIMER EQUILIBRIUM FOR THIACARBOCYANINE DYES

The monomer-dimer equilibrium for polymethine dyes, thiacarbocyanine ( 1) and its meso-substituted derivatives [9-ethyl- (2) and 9-methylsubs tituted (3)], was investigated by absorption spectra in aqueous soluti ons at 15-40 degrees C. The formation of dimers is accompanied by the appearance of the absorption band shifted by 35-45 nm to shorter wavel engths with respect to the absorption band of the monomers. Changes in fluorescence and fluorescence excitation spectra of the dyes with inc reasing their concentration show the formation of nonfluorescing dimer s. The values of the dimerization constants for dyes 3, 1, and 2 at 20 degrees C were found to be 4.4 x 10(3), 3.8 x 10(4), and 2.8 x 10(5) l/mol, respectively For 1 and 2, the dimerization enthalpies were dete rmined to be -44.0 and -72.3 kJ/mol, respectively, and the dimerizatio n entropies were calculated to be -62.5 and -142.3 J mol(-1) K-1, resp ectively.