Abstract
5-Aminoquinoline and 8-aminoquinoline fluoresce moderately to weakly in low dielectric media but not in strongly hydrogen-bonding or acidic aqueous media. At low temperatures and in rigid media moderately intense fluorescence is observed in aqueous, media at all pH values. Quenching of the aminoquinoline fluorescences in fluid media is believed to be due primarily to hydrogen bonding in the lowest excited singlet state. The fluorescences of the aminoquinolines arise from the 1L a states in the respective molecules. Calculation of approximate excited singlet state p K a * values indicates that upon excitation to the 1L a state the amino group becomes a weaker base while the heterocyclic ring nitrogen becomes a stronger base. Phosphorescence is observed for the doubly protonated cations of both aminoquinolines but not for the singly protonated or neutral species. Both neutral species exhibit delayed fluorescence at 77° K.
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