The excited-state dynamics of magnesium octaethylporphyrin studied by femtosecond time-resolved four-wave-mixing

Femtosecond four-wave-mixing spectroscopy was used to study the initial molecular relaxation processes of magnesium octaethylporphyrin, an effective modelling compound for porphyrin and porphyrin-like compounds in nature. By choosing the excitation- and probe-wavelengths to be resonant with the S1 and S2 excited states, a detailed mapping of the relaxation dynamics within the excited-state manifold is achieved. This data allows the placing of an upper limit of ∼100 fs for the Q01 → Q00 electronic relaxation and due to the lower time resolution in experiments with excitation at 400 nm, of less than 350 fs for the Soret → Q00 population flow. After thermal equilibration, the S1 excited Q00-state is depopulated on a time scale much longer than that accessible to our measurements. However, within the Q00-band an intrastate relaxation is observed, which proceeds on the time scale of a few hundred femtoseconds depending on whether lower or higher excited vibrational levels are involved in the respective relaxational process.