title: 2,4,5,7-Tetranitrofluorescein in solutions: novel type of tautomerism in hydroxyxanthene series as detected by various spectral methods.
abstract: 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 4.00M KCl. The pK(a) values in DMSO and 90 mass% (CH3)2CO as well as the 'apparent'pK(a)(a) 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 lambda(max) 522-525 nm and E(max) approximately (60-62)x10(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 1.17x10(3)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.
authors.affiliation: Department of Physical Chemistry, V.N. Karazin Kharkov National University, Svoboda sq. 4, 61077 Kharkov, Ukraine. mchedlov@univer.kharkov.ua
authors.forename: Nikolay O
authors.initials: NO
authors.lastname: McHedlov-Petrossyan
authors.forename: Natalya A
authors.initials: NA
authors.lastname: Vodolazkaya
authors.forename: Yuriy N
authors.initials: YN
authors.lastname: Surov
authors.forename: Denis V
authors.initials: DV
authors.lastname: Samoylov
mesh_terms: D000096:Acetone; D004121:Dimethyl Sulfoxide; D019793:Fluorescein / Q000737:chemistry*; D005452:Fluoresceins / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007477:Ions / Q000737:chemistry; D007536:Isomerism; D008823:Micelles; D015394:Molecular Structure; D009574:Nitro Compounds / Q000737:chemistry*; D012996:Solutions / Q000737:chemistry; D013057:Spectrum Analysis; D013816:Thermodynamics; D014966:Xanthenes / Q000737:chemistry*
chemical_list: C504583:2,4,5,7-tetranitrofluorescein; D005452:Fluoresceins; D007477:Ions; D008823:Micelles; D009574:Nitro Compounds; D012996:Solutions; D014966:Xanthenes; D000096:Acetone; D019793:Fluorescein; D004121:Dimethyl Sulfoxide