Molecular twisting and relaxation in the excited state of triarylpyrylium cations

Excited-state twisting and relaxation of triarylpyrylium cations with various substituents attached to different parts of the molecule were studied by means of femtosecond pump-probe absorption spectroscopy and modeled numerically. The model was based on calculations of the population evolution on the excited- and ground-state potential surfaces, which are significantly different for nonstabilized and stabilized states because of the essential angular dependence of the stabilization energy. The modeling shows that a broad population distribution along the twisting angle in the ground state is transferred to the excited state, causing strong fluorescence broadening, while competition between the excited-state twisting and solvation determines a subsequent reaction path. The internal conversion rate is determined by the energy gap law and, depending on the attached substituents, is governed either by twisting or by solvation processes.