Nonlinear absorption in modified porphyrins

We study one free base and seven metallo- octabromotetraphenylporphyrins by several techniques. In a pico-second pump-probe experiment, we monitor the transient transmission of each sample up to 11 ns after it is irradiated by an intense laser pulse. Combined with the results from time-resolved fluorescence spectroscopy, we propose a simple model to interpret the transmission data. We attribute the reduction in the transmission to triplet state absorption and extract the triplet state absorption cross-sections, as well as the lifetimes from the time dependent transmission data. In a separate experiment where the transmission of a nanosecond laser pulse is measured with various input energies, our measurement in the cross- sections predicts the correct optical limiting behavior. We assess the overall optical limiting performance of all 8 samples by direct comparison with C60 at the same ground state transmission.