In this article, we examined the fluorescent properties of 4′- and 5′-aminofluorescein, unsubstituted fluorescein, and its 4′-nitro derivative in a set of solvent systems. Fluorescence lifetimes, quantum yields, time-resolved fluorescence spectra, and quantum chemical calculations allowed clarifying the reasons of the emitting properties in this dye series. In water, the dianions R2– of aminofluoresceins are practically nonfluorescent; in alcohols, the quantum yields are low. In dimethylsulfoxide (DMSO), acetonitrile, and other non-hydrogen bond donor solvents, the bright fluorescence of R2– ions is quenched either on adding small amounts of water which hydrate the carboxylate group or under conditions of protonation of this group (COO– → COOH). The last observation is possible owing to the peculiarities of the tautomerism of the 5′-aminofluorescein monoanion, HR–, which exists in DMSO as an equilibrium mixture of a colorless lactone and colored “phenolate” tautomer with an ionized xanthene moiety and unionized carboxylic group. In contrast, the R2– anion of 4′-nitrofluorescein demonstrates spectral behavior different from that of the amino derivatives. It practically does not emit in aprotic solvents; however, in alcohols or water media, its quantum yield increases to some extent. Such changing spectral properties are explained in terms of the excited-state interfragmental charge transfer.

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.

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This paper is aimed to estimate the transmittance of the electronic effects in the fluorescein molecule between the phthalic acid residue and the xanthene moiety. Despite the almost orthogonal orientation of the latter with respect to the rest of the molecule, some influence of substituents in this 9-aryl ring on the dissociation of the hydroxyl group of the hydroxyxanthene cannot be ruled out. In order to reveal this (possible) effect, we blocked the carboxylic group via esterification. The reason of using dimethyl sulfoxide as solvent was the high Hammett’s “rho” constant for phenolic group. The pKa values of eight methyl or ethyl esters of 3'-, 4'-, and 5'-nitro and amino fluoresceins were determined in benzoate and salicylate buffer solutions using the spectrophotometric method. For nitro derivatives, the dissociation constants of the cationic forms of the dyes were also determined in diluted p-toluenesulfonic acid. The study reveals increasing in the pKa by 0.2–0.3 units in the case of amino derivatives, while the nitro group decreases the pKas by 0.2–0.8. Also, the position of the substituent is of importance; the 5'-substituents display the least influence. In addition, the pKa values of methyl and ethyl esters of eosin (2,4,5,7-tetrabromofluorescein) were determined in order to clarify the influence of the alkyl group. The absorption maxima and molar absorptivities of the anions, as well as the pKas in DMSO coincide within the margin of errors.