Ultrafast Relaxation Processes of Triarylpyrylium Cations

The time evolution of the fluorescence and the photoinduced absorption spectra of four triarylpyrylium cations, differing in the number of dodecyl chains attached to the chromophore, was studied by means of femtosecond fluorescence upconversion and picosecond pump−probe absorption techniques. The dependence on solvent viscosity was also examined. The results are rationalized in terms of excited and ground-state relaxation dynamics and may be differentiated into intramolecular conformational changes and intermolecular solute−solvent contributions. The dynamic fluorescence shift is related to solvent relaxation, whereas the fluorescence intensity decay is attributed to molecular twisting acting in an intricate manner. First, the geometrical relaxation reduces rapidly the fluorescence oscillator strength, but it also greatly enhances the nonradiative S<INF>0</INF> ← S<INF>1</INF> internal conversion rate. Molecular back-twisting combined with vibrational relaxation in the ground state is shown to be the last and the slowest process of the reaction.