Protolytic Equilibrium of Tetra- And Pentanitrofluoresceins in a Binary Solvent Acetonitrile – Dimethyl Sulfoxide (Mass Ratio 96 : 4)

Abstract

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.