Acridin-9-amines and their derivatives substituted at the exocyclic nitrogen atom can co-exist in amino (1) and imino (2) tautomeric forms. Electron-donating substituents (R) cause the amino form to be more stable, but electron-attracting ones cause the imino tautomer to prevail. Crystals of acridin-9-amines substituted with -t-Bu or -CH₂CH₂Cl at the exocyclic nitrogen atom consist of molecules of the amino tautomer, but those with -OCH₃ or -COCl₃ - contain molecules of the imino form. NMR investigations have shown that if R is -OCH₃ or -NH₂, the respective acridin-9-amines occur in the imino tautomeric form in various media. When R is -CH₂CH₂Cl or 5-methylpyridin-2-yl, the preference for a particular tautomeric form depends on the properties of the medium. The results of stationary absorption and fluorescence, time-resolved fluorescence investigations and computational studies have demonstrated that if -CH₂CH₂Cl is a substituent, it is mainly the imino form that absorbs radiation, whereas the amino form fluoresces. When the substituent is 5-methylpyridin-2-yl, the absorbing form is mainly the amino tautomer, and the emitting form is the imino one. As tautomeric forms of the latter two compounds that absorb radiation do not emit it, is probable that H-atom transfer takes place in the excited state. The spectral properties of these acridin-9-amines, both NMR and UV-Vis, thus depend markedly on the properties of the solvent, which makes them potent indicators of the properties of liquid phases. On the other hand, the tautomerism of acridin-9-amines is an interesting subject for investigating the fundamental behavior of chemical systems.

(Study supported by state funds for scientific research under DS/8000-4-0026-6)

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1. Physicochemical Properties of Substituted 9-(Phenoxycarbonyl) Acridinium Cations - an Important Systems Used in Luminescent Assays