Fluorescence quenching of auramine in fluid solutions: A femtosecond spectroscopy study

The quenching of Auramine fluorescence in ethanol is studied by two ultrafa st spectroscopy techniques. The gain band, probed by transient absorption s pectroscopy, vanishes in a few picoseconds, while a transient absorption ba nd rises and the ground-state repopulation is delayed. In up-conversion exp eriments, nonexponential wavelength-dependent fluorescence decays are obser ved. The average decay times increase with the wavelength and the reconstru cted instantaneous spectrum exhibits a few hundred-wavenumber red shift and a broadening while its intensity drops. The previously proposed relaxation model, involving a barrierless internal twisting motion toward a transient dark state, is further examined. In particular, the extinction coefficient s of the transient state are extracted from the differential absorption spe ctra. The band is found to lie in the same wavelength range as the dimethyl aniline cation radical. This result is discussed as a possible support for an internal twisting process involving a charge shift.