Confirmation of iminoacridan structure of singly protonated 9-aminoacridine.

The occurrence of the second absorption band of singly protonated 9-aminoacridine, which arises from an acridinium ring-localized transition, at anomalously short wavelength indicates the disruption of the aromaticity of the central ring of the singly charged cation. The shifting of the two longest wavelength absorption bands and the fluorescence band of 10-methyl-9-aminoacridine monocation, a model of singly protonated 9-aminoacridine which is constrained to dissociate from the exocyclic nitrogen atom, to shorter wavelengths, upon dissociation, indicates the imine-like nature of the 10-methyl derivative. Thes observations support the conclusion that the predominant site of positive charge in singly protonated 9-aminoacridine is the exocyclic nitrogen atom, even though the heterocyclic nitrogen atom is the site of protonation of the neutral molecule.