New fluorescein dyes with unusual properties: Tetra- and pentanitrofluoresceins

Abstract

Among various fluorescein dyes, nitro derivatives are less studied. At the same time, these compounds have some unusual properties. This article reports on the behavior of new dyes of this series in solution. Two compounds, 2,4,5,7,4′- and 2,4,5,7,5′-pentanitrofluoresceins, were synthesized, and their molecular structures in the solid state and solution were characterized by X-ray diffraction analysis and NMR spectroscopy. Electronic spectroscopy in dimethyl sulfoxide, DMSO, proves that doubly charged anions of these compounds, R2–, exist in solution in the form of lactone structures, which is not characteristic of other fluorescein dyes. These lactones are yellow (λmax = 420–430 nm) due to the nitrophenolate absorption. On the contrary, singly charged anions are represented predominantly by fluorescent and intensely colored particles with molar absorptivities of (92–96)× 103 M−1cm−1 at λmax = 525 nm, while H2R molecular forms are colorless lactones. The dyes are relatively strong acids; pKa1 and pKa2 values ​​determined in DMSO do not exceed 5 units. It is important to note that in this solvent, as in other media, the pH range of the predominance of HR– monoanions is very narrow. The protolytic equilibrium state of 2,4,5,7-tetranitrofluorescein is similar. The product of the pyran ring rupture of the last-named dye was isolated and identified; this allowed to rationalize the color changes of the parent dye in aqueous solutions within a wide pH range. Of particular interest is the methyl ester of 2,4,5,7-tetranitrofluorescein. The esterification of the carboxyl group avoids lactonization. In contrast to the initial dye, this compound exists as a stable singly charged anion in a wide pH range in all solvents except nonpolar ones, and its fluorescence is very sensitive to the hydrogen bonding ability of the solvent.