Photophysics of organic materials exhibiting strong two-photon and excited state absorption

We report photophysical measurements and application of an effective three-photon absorption model that characterize the two-photon and excited state absorption in organic D-π-A chromophores. The key parameter is an effective three-photon absorption coefficient that depends on the intrinsic molecular two-photon absorption cross section and excited state photophysical properties. We measure all of these molecular parameters independently in a variety of experiments and then compare the model predictions with nanosecond nonlinear absorption measurements. We find excellent agreement with the data using only experimentally measured molecular quantities and no free parameters. We conclude that excited state absorption from both singlet and triplets states is the dominant contribution to the nonlinear transmittance loss in the nanosecond regime, and that the chief role of two-photon absorption in this regime is to populate the excited triplet state.