AGGREGATION OF AZO DYES CONTAINING PENTAFLUOROANILINE AS A DIAZO COMPONENT IN AQUEOUS-SOLUTIONS

The aggregation of monoazo sulfonic dyes containing pentafluoroaniline as a diazo component in aqueous solutions was investigated by means of 19F NMR and electronic absorption spectroscopy. The five fluorine atoms attached to the benzene ring gave a NMR spectrum with one doublet and two triplets. With an increase in dye concentration, all the signals of the fluorines exhibited progressive shifts to lower magnetic field, indicating that the fluorine atoms in the aggregate are located outside the aromatic ring of an adjacent dye molecule. The direction of the shifts did not vary from dye to dye, but the extent did, depending on the number of sulfonic groups. The results are used to suggest a molecular model of dye aggregation. The visible absorption spectra of the dye solutions showed a single monomer/dimer equilibrium. This result permitted the calculation of aggregation constants, K. These K values agreed quite well with those determined by 19F NMR spectroscopy. The aggregation constants depended on the presence of fluorine atoms and the number of sulfonic groups. The mechanism of these effects on the aggregation is also discussed in detail. © 1990 American Chemical Society.