Photoluminescence of 6- and 7-aminoquinolines

The fluorescences of 6- and 7-aminoquinoline and 6-dimethylaminoquinaldine were studied in media of different acidities, polarities, hydrogen-bonding capabilities and temperatures. The excited state acidities and solvent dependences of the fluorescences of these compounds are typical of aminoquinolines substituted in the homocyclic ring. The acidities of the doubly protonated cations are so great in the lowest excited singlet state, however, that prototropic dissociation in the excited state occurs even in rigid media at 195°K. The lowest triplet state of the singly charged cation of 6-aminoquinoline is found to be nearly degenerate with the lowest excited singlet state of the same molecule. This is attributed to the great charge-transfer component of the S₁←S₀ transition which is not present in the T₁←S₀ transition, and the interaction of the S₁ state with the polar solvent.