In this paper, the acid-base and tautomeric equilibria of four nitrofluorescein dyes, 2,4,5,7-tetranitrofluorescein, 2,4,5,7,4’-pentanitrofluorescein, 2,4,5,7,5’-pentanitrofluorescein, and 2,4,5,7-tetranitrofluorescein methyl ester, were studied. As reaction media, a binary solvent acetonitrile – dimethyl sulfoxide (96 : 4 by mass) was used. The acidity scale in this solvent was established previously. The indices of the dissociation constants of the dyes were determined using the spectrophotometric method.
Interpreting the values requires an understanding of the state of tautomeric equilibria. The behavior of these compounds differs significantly from that of other fluorescein dyes, e.g., halogen derivatives. In the case of the first three compounds, i.e., for dyes with a free carboxylic group, the lactonic structure is predominant not only for the neutral form, but even for the double-charged anion. The single-charged anionic form exists as an equilibrium mixture of a colored (and fluorescent) tautomer and an almost colorless lactone. The fourth compound with esterified carboxylic group exhibits extreme stability in its anionic form.
Evaluation of the tautomerization constants made it possible to calculate the microscopic equilibrium constants of the stepwise dissociation of dye lactones, k1L and k2L. The consideration of the difference (pk2L – pk1L) allowed estimating the effective relative permittivity of the space between the ionizing groups basing on the Bjerrum – Kirkwood – Westheimer equation. Tautomerism of anions was discussed from the point of view of stabilization of symmetric structures.
Mchedlov-Petrossyan N. O., Vodolazkaya N. A. Protolytic equilibria in organized solutions: ionization and tautomerism of fluorescein dyes and related indicators in cetyltrimethylammonium chloride micellar solutions at high ionic strength of the bulk phase. Liquids. 2021. 1: 1–24.
Mchedlov-Petrossyan N. O., Steinbach K., Vodolazkaya N. A., Samoylov D. V., Shekhovtsov S. V., Omelchenko I. V., Shishkin O. V. The molecular structure of anionic species of 2,4,5,7-tetranitrofluorescein as studied by ESI, NMR, and X-ray techniques. Coloration Technology. 2018. 134(5): 390–399.
Shekhovtsov S. V., Mchedlov-Petrossyan N. O., Kamneva N. N., Gromovoy T. Yu. New orange dyes: Nitroderivatives of sulfonefluorescein. Kharkov University Bull. 2014. 24: 7–18.
Mchedlov-Petrossyan N.O., Vodolazkaya N.A., Surov Yu.N., Samoylov D.V. 2,4,5,7–Tetranitrofluorescein in solutions: novel type of tautomerism in hydroxyxanthene series as detected by various spectral methods. Spectrochim. Acta. Part A. Mol. and Biomol. Spectrosc. 2005. 61(11–12): 2747–2760.
Mchedlov-Petrosyan N. O., Vodolazkaya N. A., Martynova V. P., Samoilov D. V., El’tsov A. V. Protolytic properties of thiofluorescein and its derivatives. Russ. J. Gen. Chem. 2002. 72(5): 785–792.
Samoilov D. V., Mchedlov-Petrosyan N. O., Martynova V. P., El’tsov A. V. Protolytic equilibria of fluorescein nitro derivatives. Russ. J. Gen. Chem. 2000. 70(8): 1259–1271.
Gensh K. V., Zevatskii Yu. E., Samoylov D.V., Shekhovtsov S.V., Lebed A. V., Goga S.T., Mchedlov-Petrossyan N.O. Ionic equilibrium in mixtures of polar protophobic and protophilic non-hydrogen bond donor solvents: Acids, salts, and indicators in acetonitrile with 4 mass % dimethylsulfoxide. J. Mol. Liquids. 2020. 322: 114560.
Mchedlov-Petrossyan N.O., Filatov D.Yu., Goga S.T., Lebed A.V. Ionic equilibrium in mixtures of protophobic and protophilic polar non-hydrogen bond donor solvents: Acids, salts, and indicators in acetone containing 5 mol % DMSO. J. Phys. Org. Chem. 2010. 23(5): P. 418–430.
Kolthoff I.M., Chasntooni M.K., Bhowmik S. Acid-base properties of mono- and dinitrophenols in acetonitrile. J. Amer. Chem. Soc. 1966. 88(23): 5430–5438.
Mchedlov-Petrossyan N.O. Kukhtik V.I. , Bezugliy V.D. Dissociation, tautomerism and electroreduction of xanthene and sulfonephthalein dyes in N,N-dimethylformamide and other solvents. J. Phys. Org. Chem. 2003. 16: 380–397.
Vereshchagin A.N. Inductive Effect. Nauka: Moscow. 1987: 328.
Lebed A. V., Biryukov A. V., Mchedlov-Petrossyan N. O. A quantum-chemical study of tautomeric equilibria of fluorescein dyes in DMSO. Chem. Heterocycl. Comp. 2014. 50(3): 336–348.