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

• Published in: Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy Molecular and biomolecular spectroscopy, 2005-09, Vol.61 (11), p.2747-2760
• Main Authors: Mchedlov-Petrossyan, Nikolay O., Vodolazkaya, Natalya A., Surov, Yuriy N., Samoylov, Denis V.
• Format: Article
• Language: English
• Subjects: 2,4,5,7-Tetranitrofluorescein; Acetone; Dimethyl Sulfoxide; Fluorescein - chemistry; Fluoresceins - chemistry; Fluorescence; Hydrogen-Ion Concentration; Ions - chemistry; Isomerism; Lactonic anions; Micellar media; Micelles; Molecular Structure; Nitro Compounds - chemistry; Organic solvents; Protolytic equilibria; Solutions - chemistry; Spectrum Analysis; Thermodynamics; Vis- and IR-spectroscopy; Xanthenes - chemistry
• ISSN: 1386-1425
• DOI: 10.1016/j.saa.2004.09.030

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 p K a values in DMSO and 90 mass% (CH 3) 2CO as well as the ‘apparent’ p K a a values of the substance in micellar media were determined spectrophotometrically. The neutral (molecular) form H 2R 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 R 2−. Contrary to the unsubstituted fluorescein, and like 2,4,5,7-tetrabromofluorescein (eosin), the monoanion HR − of TNF with λ max 522–525 nm and E max ∼(60–62) × 10 3 dm 3 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 × 10 3 cm −1, fluorescence lifetime equals 2.3 ns. An extremely expressed trend to dianion–lactone formation of R 2− 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.