Dual fluorescence of the quinolinium cation

Abstract

pH and Hammett acidity dependences of the blue fluorescences of the quinolinium ion and some related species indicate that the thermally relaxed lLb and'L, states of the quinolinium ion are degenerate and that fluorescence occurs from both states. The quinolinium cation is a weaker acid in one of these states than in the ground state and a stronger acid in the other excited state. Spectral shifts upon protonation suggest that the other excited state. Spectral shifts upon protonation suggest that the L state is the more basic of the two low-lying excited singlet states. Prototropic equilibrium in the more basic excited state is not complete while the more acidic excited state equilibrium appears to be attained.