Effect of hydroxylic compounds on the photophysical properties of ellipticine and its 6-methyl derivative: The origin of dual fluorescence

Photophysical parameters of ellipticine were compared to those of its 6-methyl derivative in various solvents. Since both compounds emitted dual fluorescence in methanol and ethylene glycol, the band peaking at low energy cannot be due to a tautomer formed via solvent-assisted excited-state proton transfer but originates from photoinduced protonation by the solvent. The mechanism and kinetics of the processes undergoing in the excited-state in the presence of OH− were revealed in methanol. Addition of OH− caused quenching for the excited ellipticine, whereas back-formation of the neutral excited compound proved to be the only important photochemical reaction of the protonated species.

The long-wavelength fluorescence of ellipticine (E) in methanol is assigned to photoinduced protonation by the solvent instead of a 'quinoid-like' tautomer formation via long distance, solvent-assisted excited-state intramolecular proton transfer, which has been proposed for olivacine, a compound differing from E only in the position of a methyl group.