The molecular structure of anionic species of 2,4,5,7‐tetranitrofluorescein as studied by electrospray ionisation, nuclear magnetic resonance and X‐ray techniques

Among the fluorescein dyes, the behaviour of their nitro derivatives in solution is most unusual, and yet at the same time rarely studied. In this study, their ¹³C nuclear magnetic resonance and electrospray spectra are reported, which confirm, in concert with their ultraviolet‐visible spectra, the molecular structures of the neutral and ionic species of 2,4,5,7‐tetranitrofluorescein observed in different solvents and at various pH values. A peculiar property of this dye consists in its lactonisation of the double‐charged anion, contrary to the well‐known coloured and fluorescent dianions of fluorescein, eosin, etc. Also, this dianion‐lactone exhibits high ability for opening the pyran ring. This cleavage takes place at much lower pH values as compared with those for fluorescein and its halogen derivatives. The ruptured species can be formed as either lactone or carbinol; the latter appears as a result of the nucleophilic attack of a HO⁻ on the central carbon atom. The corresponding molecular structures are identified via the electrospray method and X‐ray analysis. The behaviour of some other nitrofluoresceins in alkaline media is also considered.