Protolytic equilibrium, light absorption and emission of 2,7-dinitro-4,5-dibromofluorescein and related dyes: Fluorescent indicators sensitive to hydrogen bonds of the solvent

Abstract

This article reports on the spectral data and protolytic equilibria parameters of a series of fluorescein halogen and nitro-halogen derivatives in solution. Absorption spectra and dissociation constants of 2,7-dinitro-4,5-dibromofluorescein (also called eosin bluish or eosin B), 2,4,5,7-tetrabromofluorescein (eosin or eosin Y), 2,7-dichlorofluorescein, and 3′,4′,5′,6′-tetrachlorofluorescein are determined in DMSO. The state of the tautomeric equilibria of molecules and anions is discussed. The behavior of the nitro-bromo derivative in solution strongly differs from other dyes due to the pronounced influence of the NO2 groups in 2 and 7 positions. As a result, the lactone tautomer predominates not only for the neutral form, H2R, but even for the double-charged anion, R2–. This kind of chain-ring tautomerism is not typical for other dyes under study. The influence of the solvent’s nature on the fluorescent properties of this dye is also of special interest. Whereas the solvatochromism of the HR– form of this compound as well as of the R– ion of its methyl ester is of medium strength, the emission intensity dramatically changes from bright fluorescence in DMSO and other non-HBD solvents to very poor in alcohols and water. Thus, these two dyes join the group of previously studied fluorescein derivatives, the fluorescence quantum yield of which is sensitive to the hydrogen bonds of the medium.