Aggregation of o,o′-dihydroxyazo dyes -: II.: Interaction of 2-hydroxy-4-nitrophenylazoresorcinol in DMSO and DMF

The aggregation behavior and tautomerism of several concentrations (10(-4)-10(-5) mol liter(-1)) of 2-hydroxy-4-nitrophenylazoresorcinol in DMSO and DMF was investigated by UV-visible spectroscopy. The electronic absorption spectra were recorded immediately after dissolving the dye sample in the solvent. The absorption spectra of the solutions were then measured on intervals through 36 days (868 h) at 22 degrees C. The lowest concentration (1x10(-5) mol liter(-1)) gave an intense electronic absorption band at lambda 404 nm that was assigned to the azo monomer form. This absorption pattern was slightly bathochromic shifted with time. Concentrations above 6x10(-5) mol liter(-1) gave two absorption bands at ca. lambda 420 and ca. lambda 520 nm. The former band was assigned to the dimeric form of the dye and the latter one to the hydrate monomeric form. The time dependence of the electronic absorption spectra of all solutions above 6x10(-5) mol liter(-1) reflected a linear exchange from the absorption band at lambda 520 nm (hypochromically shifted) to the absorption band at lambda 420 nm (hyperchromically shifted). Equilibrium between the different species was reached in about 450 h after mixing. Then, both bands were stable for ca. 100 h. After this time both bands started to reflect a hypochromic shift, indicating degradation of the absorbing species. In DMF the time dependence of the absorption spectra of 6.4x10(-5) mol liter(-1) of the dye reflected the same behavior of the dye in DMSO. However, the above mentioned bands assigned to the dimer and the hydrate monomer forms appeared at ca. lambda 470 nm and ca. lambda 550 nm, respectively. It was concluded that this concentration time dependent interaction is most probably due to the shifting of the hydrazo-azo equilibrium, caused by the shifting of the dimer-monomer equilibrium. This reaction was followed kinetically using the initial rate method. The observed kinetic profile resembles that of auto-catalyzed reactions. A mechanism was proposed to account for the observed kinetics. (C) 1999 Elsevier Science Ltd. All rights reserved.