Prototropic equilibria of some Harmala alkaloids in acid solutions: proton-induced fluorescence quenching of the monocations of harmaline and harmalol
Absorption and fluorescence spectra of harman homologues in acid solutions have shown the presence of mono-, di-, and tri-cations. Fluorescence spectra for the various cations at 77 K and their quantum yields at 298 K are reported. The acidity constants indicate that these alkaloids are more basic in the excited singlet state than in the ground state. The proton-induced fluorescence quenching rate constant for the monovalent cations is observed to be of the order of 109 dm3 mol–1 s–1 for harmaline and harmalol and considerably less for harman, harmine, and harmol.