3′-Nitro- and 3′-Aminofluoresceins: Appearance of Previously Missing Dyes

Contrary to the 4′- and 5′-nitro- and aminofluoresceins, the corresponding 3′-derivatives are practically unexplored. In this paper, we describe the synthesis and spectral properties of 3′-nitrofluorescein and 3′-aminofluorescein, as well as their methyl esters. Among other methods, X-ray analysis, 13C NMR spectroscopy, and ESI mass spectrometry made it possible to establish the molecular structure of the target compounds as well as intermediates and by-products. Some unexpected products, though in small amounts, were revealed within the course of study. Whereas the fluorescence of the double-charged R2− ion of 3′-nitrofluorescein in both aqueous and organic solvents is weak, the R2− anion of 3′-aminofluorescein in a non-hydrogen bonding donor solvent, but not in water, exhibits intensive fluorescence, analogous to the case of 4′- and 5′-aminofluoresceins. Interestingly, the λmax values in water of the R2− ions bearing an NO2 group in the 3′- and 6′-positions are 7 to 10 nm higher than those of the 4′- and 5′-nitro derivatives. The difference was also observed in dimethyl sulfoxide. This correlates with the angles between the xanthene and phthalic planes of the dyes. The dye 3′-aminofluorescein could be used as a fluorescent indicator sensitive to hydrogen bonding ability of the solvent. It could also serve as a platform for synthesizing fluorescent molecular probes for biochemical research, analogous to the very popular application of 4′- and 5′-amino derivatives.