Maintaining of the Green Fluorescence Emission of 9‑Aminoanthracene for Bioimaging Applications

The green fluorescence emission of 9-aminoanthracence (<b>9AA</b>) was maintained by controlling the oxidation of <b>9AA</b> with oxygen in the solid state and in solution. The solid-state fluorescence of <b>9AA</b> was maintained for a longer time when lauric acid was used because the equilibrium between <b>9AA</b> and 9-anthrylammonium salt (<b>9AAH</b>^<b>+</b>) inclines toward the right-hand side in the presence of an acid. A solution of <b>9AA</b> in CDCl₃, to which nitrogen had been bubbled through for 5 min, continued to emit green fluorescence for more than 3 days, whereas the fluorescence emission disappeared within 3 days for the solution that had been bubbled with oxygen for 5 min. <b>9AA</b> is oxidized by oxygen in MeOH under dark conditions to give almost nongreen fluorescent anthraquinone monoimine (<b>AQNH</b>), whereas dimerization of <b>9AA</b> occurs under UV irradiation at 365 nm, much faster than the generation of <b>AQNH</b>. These results suggest that <b>9AA</b> is oxidized by the triplet rather than the singlet oxygen in MeOH. Some of the organic molecules, proteins, and biological tissues were successfully stained with <b>9AA</b> on microscope slides within 10 min because the green fluorescence emission of <b>9AA</b> was successfully maintained in the presence of an acid and under hypoxic conditions of the used materials