2,4,5,7-Tetranitrofluorescein in solutions: novel type of tautomerism in hydroxyxanthene series as detected by various spectral methods

Stepwise dissociation and tautomerism of 2,4,5,7-tetranitrofluorescein (TNF) were studied by using vis-spectroscopy in dimethylsulfoxide (DMSO), in aqueous acetone, and in cetyl-trimethylammonium chloride (CTAC) micellar solutions at ionic strength of the bulk phase 4.00 M KCl. The pKa values in DMSO and 90 mass% (CH3)2CO as well as the ‘apparent’ pKaa values of the substance in micellar media were determined spectrophotometrically. The neutral (molecular) form H2R is found to be completely converted into the colorless lactone. Moreover, the lactonic structure, yellow due to ‘nitrophenolate’ absorption band, predominates also in the case of TNF dianion R2−. Contrary to the unsubstituted fluorescein, and like 2,4,5,7-tetrabromofluorescein (eosin), the monoanion HR− of TNF with λmax 522–525 nm and Emax ∼(60–62) × 103 dm3 mol−1 cm−1 exists mainly as a deeply and intensively colored structure with non-ionized carboxylic and ionized hydroxylic group; its fluorescence spectra in various media are registered. In 90% acetone, the Stokes shift is 1.17 × 103 cm−1, fluorescence lifetime equals 2.3 ns. An extremely expressed trend to dianion–lactone formation of R2− ion of TNF is confirmed in the systems studied. For TNF in DMSO, in aqueous acetone, in surfactant micelles, and in trichloromethane extracts of ionic associatiates with N(n-Bu)4+ and N(n-Hept)4+, the deeply colored ‘quinon–phenolate’ dianion, typical for all hydroxyxanthenes, is not registered at all. The sequence of dissociation of functional groups in solution is confirmed using IR spectroscopy in DMSO.