Fluorimetric detection of tautomeric equilibria in 8-aminoquinolines

The 1-methylquinolinium cations derived from 8-aminoquinoline and 8-amino-6-methoxyquinoline were prepared by methylation of the corresponding nitroquinolines and reduction of the nitro-compounds. The dissociation constants of the protonated species of these compounds are almost identical to those of doubly-protonated 8-aminoquinoline and 8-amino-6-methoxyquinoline, respectively, suggesting that the parent quinolines are exclusively first protonated at the ring nitrogen atom. However, the molar absorptivities of the 1-methyl derivatives at their longest-wavelength absorption maxima are substantially greater than the corresponding absorptivities of the unmethylated aminoquinolines, a result which suggests tautomerism of the singly-protonated parent quinolines, with a proportion of the population protonated at the amino group. Fluorescence spectroscopy reveals a single emission from the 8-amino-6-methoxy-1-methylquinolinium ion and two excitation-wavelength-dependent fluorescences from the 8-amino-6-methoxyquinolinium ion, confirming the occurrence of tautomerism and supporting choice of the absorptiometric approach rather than the titrimetric approach as the preferred method for the detection of tautomerism and the calculation of tautomeric equilibrium constants.