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-methylsubstituted (3)], was investigated by absorption spectra in aqueous solutions 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 wavelengths with respect to the absorption band of the monomers. Changes in fluorescence and fluorescence excitation spectra of the dyes with increasing their concentration show the formation of nonfluorescing dimers. 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 determined to be -44.0 and -72.3 kJ/mol, respectively, and the dimerization entropies were calculated to be -62.5 and -142.3 J mol(-1) K-1, respectively.