Ultrafast relaxation processes of triarylpyrylium cations

The time evolution of the fluorescence and the photoinduced absorption spec tra of four triarylpyrylium cations, differing in the number of dodecyl cha ins attached to the chromophore, was studied by means of femtosecond fluore scence upconversion and picosecond pump-probe absorption techniques. The de pendence on solvent viscosity was also examined, The results are rationaliz ed in terms of excited and ground-state relaxation dynamics and may be diff erentiated into intramolecular conformational changes and intermolecular so lute-solvent contributions. The dynamic fluorescence shift is related to so lvent relaxation, whereas the fluorescence intensity decay is attributed to molecular twisting acting in an intricate manner. First, the geometrical r elaxation reduces rapidly the fluorescence oscillator strength, but it also greatly enhances the nonradiative S-0 <-- S-1 internal conversion rate. Mo lecular back-twisting combined with vibrational relaxation in the ground st ate is shown to be the last acid the slowest process of the reaction.